The relative density of a gas to another gas. What is air density and what is it equal to under normal conditions?

One of the most important physical properties gaseous different substances is the value of their density.

DEFINITION

Density is a scalar physical quantity, which is defined as the ratio of the mass of a body to the volume it occupies.

This quantity is usually denoted by the Greek letter r or the Latin letters D and d. The unit of measurement for density in the SI system is considered to be kg/m 3 , and in the GHS - g/cm 3 . Gas density is a reference value; it is usually measured at air pressure. u.

Often, in relation to gases, the concept of “relative density” is used. This value is the ratio of the mass of a given gas to the mass of another gas taken in the same volume, at the same temperature and the same pressure, called the relative density of the first gas to the second.

For example, when normal conditions the mass of carbon dioxide in a volume of 1 liter is equal to 1.98 g, and the mass of hydrogen in the same volume and under the same conditions is 0.09 g, from which the density of carbon dioxide by hydrogen will be: 1.98 / 0.09 = 22.

Relative gas density

Let us denote the relative gas density m 1 / m 2 by the letter D. Then

Therefore, the molar mass of a gas is equal to its density relative to another gas, multiplied by the molar mass of the second gas.

Often the densities of various gases are determined in relation to hydrogen, as the lightest of all gases. Since the molar mass of hydrogen is 2.0158 g/mol, in this case the equation for calculating molar masses takes the form:

or, if we round the molar mass of hydrogen to 2:

Calculating, for example, using this equation the molar mass of carbon dioxide, the density of which for hydrogen, as indicated above, is 22, we find:

M(CO 2) = 2 × 22 = 44 g/mol.

The density of a gas can be determined independently in laboratory conditions as follows: you need to take a glass flask with a stopcock and weigh it on an analytical balance. The initial weight is the weight of the flask from which all the air has been pumped out, the final weight is the weight of the flask filled to a specific pressure with the gas being tested. The difference in mass obtained should be divided by the volume of the flask. The calculated value is the density of the gas under these conditions.

p 1 /p N ×V 1 /m×m/V N = T 1 /T N ;

because m/V 1 = r 1 and m/V N = r N , we find that

r N = r 1 ×p N /p 1 ×T 1 /T N .

The table below shows the densities of some gases.

Table 1. Density of gases under normal conditions.

Examples of problem solving

EXAMPLE 1

Exercise	The relative density of the gas for hydrogen is 27. The mass fraction of the hydrogen element in it is 18.5%, and the boron element is 81.5%. Determine the formula of the gas.
Solution	The mass fraction of element X in a molecule of composition NX is calculated using the following formula: ω (X) = n × Ar (X) / M (HX) × 100%. Let us denote the number of hydrogen atoms in the molecule by “x” and the number of boron atoms by “y”. Let's find the corresponding relative atomic masses of the elements hydrogen and boron (the values ​​of the relative atomic masses taken from D.I. Mendeleev's Periodic Table are rounded to whole numbers). Ar(B) = 11; Ar(H) = 1. We divide the percentage content of elements into the corresponding relative atomic masses. Thus we will find the relationship between the number of atoms in the molecule of the compound: x:y = ω(H)/Ar(H) : ω (B)/Ar(B); x:y = 18.5/1: 81.5/11; x:y = 18.5: 7.41 = 2.5: 1 = 5: 2. This means that the simplest formula for the compound of hydrogen and boron is H 5 B 2 . The molar mass of a gas can be determined using its hydrogen density: M gas = M(H 2) × D H2 (gas); M gas = 2 × 27 = 54 g/mol. To find the true formula of the compound of hydrogen and boron, we find the ratio of the resulting molar masses: M gas / M(H 5 B 2) = 54 / 27 = 2. M(H 5 B 2) = 5 × Ar(H) + 2 × Ar(B) = 5 × 1 + 2 × 11 = 5 + 22 = 27 g/mol. This means that all indices in the formula H 5 B 2 should be multiplied by 2. Thus, the formula of the substance will look like H 10 B 4.
Answer	Gas formula - H 10 B 4

EXAMPLE 2

Exercise	Calculate the relative density of carbon dioxide CO 2 in air.
Solution	In order to calculate the relative density of one gas from another, the relative molecular mass of the first gas must be divided by the relative molecular mass of the second gas. The relative molecular weight of air is taken to be 29 (taking into account the content of nitrogen, oxygen and other gases in the air). It should be noted that the concept of “relative molecular mass of air” is used conditionally, since air is a mixture of gases. D air (CO 2) = M r (CO 2) / M r (air); D air (CO 2) = 44 / 29 = 1.52. M r (CO 2) = A r (C) + 2 × A r (O) = 12 + 2 × 16 = 12 + 32 = 44.
Answer	The relative density of carbon dioxide in air is 1.52.

Instructions

In order to cope with the problem, it is necessary to use formulas on relative density:

First, find the relative molecular weight of ammonia, which can be calculated from the D.I. table. Mendeleev.

Ar (N) = 14, Ar (H) = 3 x 1 = 3, hence
Mr (NH3) = 14 + 3 = 17

Substitute the obtained data into the formula to determine the relative density in air:
D (air) = Mr (ammonia) / Mr (air);
D (air) = Mr (ammonia) / 29;
D (air) = 17/29 = 0.59.

Example No. 2. Calculate the relative density of ammonia to hydrogen.

Substitute the data into the formula to determine the relative density of hydrogen:
D (hydrogen) = Mr (ammonia) / Mr (hydrogen);
D (hydrogen) = Mr (ammonia)/ 2;
D (hydrogen) = 17/ 2 = 8.5.

Hydrogen (from the Latin “Hydrogenium” - “generating water”) is the first element of the periodic table. Widely distributed, it exists in the form of three isotopes - protium, deuterium and tritium. Hydrogen is a light, colorless gas (14.5 times lighter than air). When mixed with air and oxygen, it is highly explosive. Used in the chemical and food industries, as well as rocket fuel. Research is underway into the possibility of using hydrogen as fuel for automobile engines. Density hydrogen(like any other gas) can be determined different ways.

Instructions

Firstly, based on the universal definition of density - the amount of substance per unit volume. If it is in a sealed vessel, the density of the gas is determined simply by the formula (M1 – M2)/V, where M1 is the total mass of the vessel with gas, M2 is the mass of the empty vessel, and V is the internal volume of the vessel.

If you need to determine the density hydrogen, having such initial data as , here the universal equation of state of an ideal gas, or the Mendeleev-Clapeyron equation, comes to the rescue: PV = (mRT)/M.
P – gas pressure
V – its volume
R – universal gas constant
T – gas temperature in Kelvin
M – molar mass of gas
m is the actual gas mass.

An ideal gas is considered to be a mathematical gas in which the potential energy of the molecules compared to their kinetic energy can be neglected. In the ideal gas model, there are no forces of attraction or repulsion between molecules, and collisions of particles with other particles or the walls of a vessel are absolutely elastic.

Of course, neither hydrogen nor any other gas is ideal, but this model allows calculations with fairly high accuracy at temperatures close to atmospheric pressure and room temperature. For example, given the task: find the density hydrogen at a pressure of 6 and a temperature of 20 degrees Celsius.

First, convert all the original values ​​to the SI system (6 atmospheres = 607950 Pa, 20 degrees C = 293 degrees K). Then write the Mendeleev-Clapeyron equation PV = (mRT)/M. Convert it as: P = (mRT)/MV. Since m/V is density (the ratio of the mass of a substance to its volume), you get: density hydrogen= PM/RT, and we have all the necessary data for the solution. You know the pressure value (607950), temperature (293), universal gas constant (8.31), molar mass hydrogen (0,002).

Substituting this data into the formula, you get: density hydrogen under given conditions of pressure and temperature is 0.499 kg/cubic meter, or approximately 0.5.

Sources:

• how to find the density of hydrogen

Density- this is one of the characteristics of a substance, the same as mass, volume, temperature, area. It is equal to the ratio of mass to volume. The main task is to learn how to calculate this value and know what it depends on.

Instructions

Density is the numerical ratio of mass to volume of a substance. If you want to determine the density of a substance, and you know its mass and volume, finding the density will not be difficult for you. The easiest way to find the density in in this case- this is p = m/V. It is in kg/m^3 in the SI system. However, these two values ​​are not always given, so you should know several ways in which density can be calculated.

Density It has different meanings depending on the type of substance. In addition, density varies with salinity and temperature. As the temperature decreases, the density increases, and as the degree of salinity decreases, the density also decreases. For example, the density of the Red Sea is still considered high, but in the Baltic Sea it is already lower. Have you all noticed that if you add water to it, it floats up. All this happens due to the fact that it has a lower density than water. Metals and stone substances, on the contrary, sink, since their density is higher. Based on the density of bodies, their swimming was determined.

Thanks to the theory of floating bodies, according to which one can find the density of a body, water, the volume of the entire body and the volume of its immersed part. This formula looks like: Vimmer. parts / V body = p body / p liquid. It follows that the density of the body can be found as follows: p body = V submersible. parts * p liquid / V body. This condition is met based on the tabular data and the specified volumes V immersed. parts and V of the body.

Video on the topic

Tip 4: How to calculate the relative molecular mass of a substance

Relative molecular weight is a dimensionless quantity that shows how many times the mass of a molecule is greater than 1/12 the mass of a carbon atom. Accordingly, the mass of a carbon atom is 12 units. The relative molecular mass of a chemical compound can be determined by adding up the masses of the atoms that make up the molecule of the substance.

You will need

• - pen;
• - paper for notes;
• - calculator;
• - Mendeleev table.

Instructions

Find in the periodic table the cells of the elements that make up this molecule. The relative atomic mass (Ar) values ​​for each substance are indicated in the lower left corner of the cell. Rewrite them, rounding to the nearest whole number: Ar(H) – 1; Ar(P) – 31; Ar(O) – 16.

Determine the relative molecular mass of the compound (Mr). To do this, multiply the atomic mass of each element by the number of atoms in . Then add up the resulting values. For orthophosphoric acid: Mr(h3po4) = 3*1 + 1*31 + 4*16 = 98.

Relative molecular mass is numerically the same as the molar mass of the substance. Some tasks use this connection. Example: a gas at a temperature of 200 K and a pressure of 0.2 MPa has a density of 5.3 kg/m3. Determine its relative molecular weight.

Use the Mendeleev-Cliperon equation for an ideal gas: PV = mRT/M, where V is the gas volume, m3; m – mass of a given volume of gas, kg; M – molar mass of gas, kg/mol; R – universal gas constant. R=8.314472 m2kg s-2 K-1 Mol-1; T – gas, K; P - absolute pressure, Pa. Express the molar mass from this relationship: M = mRT/(PV).

As is known, densities: p = m/V, kg/m3. Substitute it into the expression: M = pRT/P. Determine the molar mass of the gas: M = 5.3*8.31*200/(2*10^5) = 0.044 kg/mol. Relative molecular weight of the gas: Mr = 44. You can assume it is carbon dioxide: Mr(CO2) = 12 + 16*2 = 44.

Sources:

• calculate relative molecular weights

In chemical laboratories and during chemical experiments At home, it is often necessary to determine the relative density of a substance. Relative density is the ratio of the density of a particular substance to the density of another under certain conditions or to the density of a reference substance, which is distilled water. Relative density is expressed as an abstract number.

You will need

• - tables and reference books;
• - hydrometer, pycnometer or special scales.

Instructions

The relative density of substances in relation to the density of distilled water is determined by the formula: d=p/p0, where d is the desired relative density, p is the density of the substance under study, p0 is the density of the reference substance. The last parameter is tabular and defined quite accurately: at 20°C water has a density of 998.203 kg/cub.m, and it reaches its maximum density at 4°C - 999.973 kg/cub.m. Before making calculations, do not forget that p and p0 must be expressed in the same units.

In addition, the relative density of a substance can be found in physical and chemical reference books. The numerical value of the relative density is always equal to the relative specific gravity the same substance under the same conditions. Conclusion: use relative tables specific gravity just as if they were relative density tables.

When determining relative density, always take into account the temperature of the test and reference substances. The fact is that the density of substances decreases with and increases with cooling. If the temperature of the test substance differs from the standard, make a correction. Calculate it as the average change in relative density per 1°C. Look for the necessary data using temperature correction nomograms.

To quickly calculate the relative density of liquids in practice, use a hydrometer. To measure relative and dry substances, use pycnometers and special scales. A classic hydrometer is a glass tube that expands at the bottom. At the lower end of the tube there is a reservoir or a special substance. On the top of the tube there are divisions showing the numerical value of the relative density of the substance under study. Many hydrometers are additionally equipped with thermometers for measuring the temperature of the substance under study.

Avogadro's law

The distance between the molecules of a gaseous substance depends on external conditions: pressure and temperature. Under the same external conditions, the spaces between the molecules of different gases are the same. Avogadro's law, discovered in 1811, states: equal volumes of different gases under the same external conditions (temperature and pressure) contain same number molecules. Those. if V1=V2, T1=T2 and P1=P2, then N1=N2, where V is volume, T is temperature, P is pressure, N is the number of gas molecules (index “1” for one gas, “2” for another).

First corollary of Avogadro's law, molar volume

The first corollary of Avogadro's law states that the same number of molecules of any gases under the same conditions occupies the same volume: V1=V2 with N1=N2, T1=T2 and P1=P2. The volume of one mole of any gas (molar volume) is a constant value. Let us recall that 1 mole contains Avogadro's number of particles – 6.02x10^23 molecules.

Thus, the molar volume of a gas depends only on pressure and temperature. Gases are usually considered at normal pressure And normal temperature: 273 K (0 degrees Celsius) and 1 atm (760 mm Hg, 101325 Pa). Under such normal conditions, designated “n.s.”, the molar volume of any gas is 22.4 l/mol. Knowing this value, you can calculate the volume of any given mass and any given quantity gas

Second corollary of Avogadro's law, relative densities of gases

To calculate the relative densities of gases, the second corollary of Avogadro's law is used. By definition, the density of a substance is the ratio of its mass to its volume: ρ=m/V. For 1 mole of a substance, the mass is equal to the molar mass M, and the volume is equal to the molar volume V(M). Hence the gas density is ρ=M(gas)/V(M).

Let there be two gases – X and Y. Their densities and molar masses – ρ(X), ρ(Y), M(X), M(Y), related to each other by the relations: ρ(X)=M(X)/ V(M), ρ(Y)=M(Y)/V(M). The relative density of gas X to gas Y, denoted as Dy(X), is the ratio of the densities of these gases ρ(X)/ρ(Y): Dy(X)=ρ(X)/ρ(Y)=M(X)xV( M)/V(M)xM(Y)=M(X)/M(Y). The molar volumes are reduced, and from this we can conclude that the relative density of gas X to gas Y is equal to the ratio of their molar or relative molecular masses (they are numerically equal).

Gas densities are often determined in relation to hydrogen, the lightest of all gases, whose molar mass is 2 g/mol. Those. if the problem says that an unknown gas X has a hydrogen density of, say, 15 (relative density is a dimensionless value!), then finding its molar mass will not be difficult: M(X)=15xM(H2)=15x2=30 g/ mole. The relative density of the gas relative to air is often also indicated. Here you need to know that the average relative molecular weight of air is 29, and you need to multiply not by 2, but by 29.

Density is usually called such physical quantity, which determines the ratio of the mass of an object, substance or liquid to the volume they occupy in space. Let's talk about what density is, how the density of a body and a substance differs, and how (using what formula) to find density in physics.

Types of density

It should be clarified that density can be divided into several types.

Depending on the object being studied:

• The density of a body - for homogeneous bodies - is the direct ratio of the mass of a body to its volume occupied in space.
• The density of a substance is the density of bodies consisting of this substance. The density of substances is constant. There are special tables that indicate the density of different substances. For example, the density of aluminum is 2.7 * 103 kg/m3. Knowing the density of aluminum and the mass of the body that is made of it, we can calculate the volume of this body. Or, knowing that the body consists of aluminum and knowing the volume of this body, we can easily calculate its mass. We will look at how to find these quantities a little later, when we derive a formula for calculating density.
• If a body consists of several substances, then to determine its density it is necessary to calculate the density of its parts for each substance separately. This density is called the average density of the body.

Depending on the porosity of the substance of which the body is composed:

• True density is the density that is calculated without taking into account voids in the body.
• Specific gravity - or apparent density - is that which is calculated taking into account the voids of a body consisting of a porous or crumbly substance.

So how do you find density?

Formula for calculating density

The formula to help find the density of a body is as follows:

• p = m / V, where p is the density of the substance, m is the mass of the body, V is the volume of the body in space.

If we calculate the density of a particular gas, the formula will look like this:

• p = M / V m p - gas density, M - molar mass of gas, V m - molar volume, which under normal conditions is 22.4 l/mol.

Example: the mass of a substance is 15 kg, it occupies 5 liters. What is the density of the substance?

Solution: substitute the values ​​into the formula

• p = 15 / 5 = 3 (kg/l)

Answer: density of the substance is 3 kg/l

Density units

In addition to knowing how to find the density of a body and substance, you also need to know the units of measurement of density.

• For solids- kg/m 3, g/cm 3
• For liquids - 1 g/l or 10 3 kg/m 3
• For gases - 1 g/l or 10 3 kg/m 3

You can read more about density units in our article.

How to find density at home

In order to find the density of a body or substance at home, you will need:

1. Scales;
2. Centimeter if the body is solid;
3. A vessel if you want to measure the density of a liquid.

To find the density of a body at home, you need to measure its volume using a centimeter or vessel, and then put the body on the scale. If you are measuring the density of a liquid, be sure to subtract the mass of the container into which you poured the liquid before making your calculations. It is much more difficult to calculate the density of gases at home; we recommend using ready-made tables that already indicate the densities of various gases.

ρ = m (gas) / V (gas)

D by Y (X) = M (X) / M (Y)

That's why:
D by air = M (gas X) / 29

Dynamic and kinematic viscosity of gas.

The viscosity of gases (the phenomenon of internal friction) is the appearance of friction forces between layers of gas moving relative to each other in parallel and at different speeds.
The interaction of two layers of gas is considered as a process during which momentum is transferred from one layer to another.
The frictional force per unit area between two layers of gas, equal to the impulse transmitted per second from layer to layer through a unit area, is determined by Newton's law:

Velocity gradient in a direction perpendicular to the direction of movement of gas layers.
The minus sign indicates that the momentum is transferred in the direction of decreasing velocity.
- dynamic viscosity.
, Where
- gas density,
- arithmetic average speed of molecules,
- average length free path of molecules.

Kinematic viscosity coefficient.

Critical gas parameters: Tcr, Pcr.

The critical temperature is the temperature above which, at any pressure, the gas cannot be converted into a liquid state. The pressure required to liquefy a gas at a critical temperature is called critical. Given gas parameters. The given parameters are dimensionless quantities that show how many times the actual parameters of the gas state (pressure, temperature, density, specific volume) are greater or less than the critical ones:

Well production and underground storage gas

Gas density: absolute and relative.

The density of a gas is one of its the most important characteristics. When talking about the density of a gas, we usually mean its density under normal conditions (that is, at temperature and pressure). In addition, the relative density of a gas is often used, which means the ratio of the density of a given gas to the density of air under the same conditions. It is easy to see that the relative density of a gas does not depend on the conditions in which it is located, since, according to the laws of the gas state, the volumes of all gases change equally with changes in pressure and temperature.

The absolute density of a gas is the mass of 1 liter of gas under normal conditions. Usually for gases it is measured in g/l.

ρ = m (gas) / V (gas)

If we take 1 mole of gas, then:

and the molar mass of a gas can be found by multiplying the density by the molar volume.

Relative density D is a value that shows how many times gas X is heavier than gas Y. It is calculated as the ratio of the molar masses of gases X and Y:

D by Y (X) = M (X) / M (Y)

Often, the relative gas densities of hydrogen and air are used for calculations.

Relative density of gas X with respect to hydrogen:

D by H2 = M (gas X) / M (H2) = M (gas X) / 2

Air is a mixture of gases, so only the average molar mass can be calculated for it.

Its value is taken to be 29 g/mol (based on the approximate average composition).
That's why:
D by air = M (gas X) / 29

Gas density B(рв, g/l) is determined by weighing (mв) a small glass flask of known volume with gas (Fig. 274, a) or a gas pycnometer (see Fig. 77), using the formula

where V is the volume of the cone (5 - 20 ml) or pycnometer.

The flask is weighed twice: first evacuated and then filled with the test gas. By the difference in the values ​​of the 2 obtained masses, the mass of the gas mв, g is determined. When filling a flask with gas, its pressure is measured, and when weighing, its temperature environment, which is taken as the temperature of the gas in the flask. The found values ​​of p and T of the gas make it possible to calculate the density of the gas under normal conditions (0 °C; about 0.1 MPa).

To reduce the correction for the loss of mass of a cone with gas in the air when weighing it as a container, a sealed cone of exactly the same volume is placed on the other arm of the balance beam.

Rice. 274. Instruments for determining gas density: flask (a) and liquid (b) and mercury (c) effluent meters

The surface of this flask is treated (cleaned) each time in exactly the same way as when weighed with gas.

During the evacuation process, the flask is slightly heated and left connected to the vacuum system for several hours, since remaining air and moisture are difficult to remove. The volume of an evacuated cone may change due to compression of the walls by atmospheric pressure. The error in determining the density of light gases from such compression can reach 1%. In some cases, the relative density dв is also determined for a gas, i.e. the ratio of the density of a given gas рв to the density of another gas, chosen as standard р0, taken at the same temperature and pressure:

where Mb and Mo are, respectively, the molar masses of the test gas B and the standard gas, for example air or hydrogen, g/mol.

For hydrogen M0 = 2.016 g/mol, therefore

From this relationship, the molar mass of the gas can be determined if it is taken as ideal.

A quick method for determining the density of a gas is to measure the duration of its flow from a small hole under pressure, which is proportional to the flow rate.

where τв and τo ~ time of flow of gas B and air, respectively.

Gas density is measured using this method using the effusion meter (Fig. 274.6) - a wide cylinder about 400 mm high, inside of which there is a vessel 5 with a base 7 equipped with holes for inlet and outlet of liquid. On vessel 5 there are two marks M1 and M2 for reading the volume of gas, the expiration time of which is observed. Valve 3 serves for gas inlet, and valve 2 for outlet through capillary 1. Thermometer 4 controls the gas temperature.

The gas density is determined by its flow rate as follows. Fill cylinder b with a liquid in which the gas is almost insoluble so that vessel 5 above mark M2 is also filled. Then, through tap 3, the liquid is squeezed out of vessel 5 with the test gas below mark M1, and all the liquid should remain in the cylinder. After this, having closed valve 3, open valve 2 and allow excess gas to escape through capillary 1. As soon as the liquid reaches the M1 mark, turn on the stopwatch. The liquid, displacing the gas, gradually rises to the M2 mark. At the moment the liquid meniscus touches the M2 mark, the stopwatch is turned off. The experiment is repeated 2-3 times. Similar operations are carried out with air, thoroughly rinsing vessel 5 from any remaining test gas. Different observations of the duration of gas outflow should not differ by more than 0.2 - 0.3 s.

If it is impossible to select a liquid for the gas being studied in which it would be slightly soluble, a mercury effusion meter is used (Fig. 274, c). It consists of a glass vessel 4 with a three-way valve 1 and a leveling vessel 5 filled with mercury. Vessel 4 is located in glass vessel 3, which serves as a thermostat. Through tap 1, gas is introduced into vessel 4, displacing mercury below mark M1. The test gas or air is released through capillary 2, raising the equalization vessel 5. More sensitive instruments for determining the density of gases are the Stok gas hydrometer (Fig. 275a) and gas scales

Alfred Stock (1876-1946) - German inorganic chemist and analyst.

In the Stok hydrometer, one end of the quartz tube is inflated into a thin-walled ball 1 with a diameter of 30 - 35 mm, filled with air, and the other is pulled back into a hair 7. A small iron rod 3 is tightly compressed inside the tube.

Rice. 275. Rod hydrometer (a) and installation diagram (b)

The tip of the cut with a ball rests on a quartz or agate support. The tube with the ball is placed in a quartz vessel 5 with a polished round stopper. Outside the vessel there is a solenoid 6 with an iron core. Using a current of varying strength flowing through the solenoid, the position of the rocker arm is aligned with the ball so that the hair 7 points exactly to the zero indicator 8. The position of the hair is observed using a telescope or microscope.

The stem hydrometer is welded to tube 2 to eliminate any vibrations.

The ball and tube are in equilibrium at a given density of the gas surrounding them. If in vessel 5 one gas is replaced by another at constant pressure, then the equilibrium will be disrupted due to a change in gas density. To restore it, it is necessary to either pull rod 3 down with electromagnet 6 when the gas density decreases, or allow it to rise up when the density increases. The amount of current flowing through the solenoid when equilibrium is reached is directly proportional to the change in density.

The device is calibrated using gases of known density. The accuracy of the Stok hydrometer is 0.01 - 0.1%, sensitivity is about UP to "7 g, the measurement range is from 0 to 4 g / l.

Installation with a Stok hydrometer. The Rod hydrometer / (Fig. 275.6) is connected to the vacuum system so that it hangs on tube 2 as if on a spring. Elbow 3 of tube 2 is immersed in a Dewar vessel 4 with a cooling mixture that allows the temperature to be maintained no higher than -80 o C for condensation of mercury vapor, if a diffusion mercury pump is used to create a vacuum in the hydrometer. Tap 5 connects the hydrometer to the flask containing the gas under study. The trap protects the diffusion pump from the influence of the test gas, and device 7 serves to accurately regulate the pressure. The entire system is connected through a tube to a diffusion pump.

The volume of gas is measured using calibrated gas berets (see Fig. 84) with a thermostatically controlled water jacket. To avoid corrections for capillary phenomena, gas 3 and compensation 5 burettes are selected of the same diameter and placed in a thermostatic jacket 4 side by side (Fig. 276). Mercury, glycerin and other liquids that poorly dissolve the gas under study are used as barrier liquids.

This device is operated as follows. First, fill the burettes with liquid to a level above tap 2, raising vessel b. Then the gas burette is connected to the gas source and it is introduced, lowering vessel b, after which valve 2 is closed. To equalize the gas pressure in burette 3 with atmospheric pressure, vessel b is brought close to the burette and set at such a height that the mercury menisci in the compensation 5 and gas burette 3 are at the same level. Since the compensation burette communicates with the atmosphere (its upper end is open), with this position of the menisci the gas pressure in the gas burette will be equal to atmospheric pressure.

Simultaneously measure Atmosphere pressure according to the barometer and the temperature of the water in the jacket 4 using a thermometer 7.

The found volume of gas is brought to normal conditions (0 °C; 0.1 MPa), using the equation for an ideal gas:

V0 and V are the volume (l) of gas reduced to normal conditions and the measured volume of gas at temperature t (°C), respectively; p - atmospheric pressure at the moment of measuring the gas volume, torr.

If the gas contains water vapor or was in a vessel above water before measuring the volume or aqueous solution, then its Volume is brought to normal conditions, taking into account the water vapor pressure p1 at the temperature of the experiment (see Table 37):

The equations are used if the atmospheric pressure when measuring the volume of gas was relatively close to 760 torr. The pressure of a real gas is always less than that of an ideal gas due to the interaction of molecules. Therefore, a correction for the nonideality of the gas, taken from special reference books, is introduced into the found value of the gas volume.

Ministry of Education and Science of the Russian Federation

Federal state budget educational institution higher professional education

"Russian State University oil and gas named after. I.M.Gubkina"

A.N. Timashev, T.A. Berkunova, E.A. Mamedov

DETERMINATION OF GAS DENSITY

Guidelines for performing laboratory work in the disciplines “Technology of operation of gas wells” and “Development and operation of gas and gas condensate fields” for students of specialties:

RG, RN, RB, MB, MO, GR, GI, GP, GF

Edited by Professor A.I. Ermolaeva

Moscow 2012

Determination of gas density.

Guidelines for carrying out laboratory work / A.N. Timashev,

T.A. Berkunova, E.A. Mamedov - M.: Russian State University of Oil and Gas named after I.M. Gubkina, 2012.

Methods for laboratory determination of gas density are described. The basis is the current GOST 17310 - 2002.

The guidelines are intended for students of oil and gas universities in the following specialties: RG, RN, RB, MB, MO, GR, GI, GP, GF.

The publication was prepared at the Department of Development and Operation of Gas and Gas-

zocondensate deposits.

Published by decision of the educational and methodological commission of the Faculty of Development

Bottoms of oil and gas fields.

	Introduction……………………………………………………………………………….
	Basic definitions……………………………………………………………….
	Density of natural gas at atmospheric pressure…………..
	Relative density of gas……………………………………….
	Density of natural gas at pressures and temperatures……….
	Laboratory methods for determining the density of natural gas....
	Pycnometric method………………………………………………………………
	Calculation formulas……………………………………………………………..
	The procedure for determining density………………………………………………………
	Calculation of gas density……………………………………………………………………
	Determination of gas density by the outflow method…………………..
	Derivation of relations for determining the density of the studied ha-
	behind………………………………………………………………………..
2.2.2. Procedure of work…………………………………………………………….
2.2.3. Processing of measurement results…………………………………..
	Control questions………………………………………………..
	Literature…………………………………………………………….
	Appendix A………………………………………………………
	Appendix B……………………………………………………….
	Appendix B………………………………………………………………………………

Introduction

Physical properties natural gases and hydrocarbon condensates are used

are used both at the design stage of development and site development

densities of natural gases, and in the analysis and control of field development,

operation of the system for collecting and preparing products from gas and gas condensate wells. One of the main physical properties to be studied is the density of gas deposits.

Since the gas composition of natural gas fields is complex,

consisting of hydrocarbons (alkanes, cycloalkanes and arenes) and non-hydrocarbons

components (nitrogen, helium and other rare earth gases, as well as acidic components

nents H2 S and CO2), there is a need for laboratory determination of density

sti gases.

In this methodological instructions calculation methods for determining

determination of gas density using a known composition, as well as two laboratory methods for determining gas density: pycnometric and the method of flow through a capillary

1. Basic definitions

1.1. Density of natural gas at atmospheric pressure

The gas density is equal to the mass M contained in a unit volume of the substance

va. There are gas densities at normal temperatures P 0.1013 mPa, T 273 K and

	standard with P 0.1013 MPa, T 293 K									under conditions, as well as under any pressure
	temperature Р and temperature Т Р, Т.									known molecular weight
	density under normal conditions is equal to
	under standard conditions

Where M is the molecular mass of the gas, kg/kmol; 22.41 and 24.04, m3/kmol – molar volume of gas, respectively, at normal (0.1013 MPa, 273 K) and standard

(0.1013 MPa, 293 K) conditions.

For natural gases consisting of hydrocarbon and non-hydrocarbon components (acidic and inert), the apparent molecular mass M k

determined by the formula

				êã/ êì î ëü,

where M i is the molecular weight of the i-th component kg/kmol; n i is the mole percentage of the i-th component in the mixture;

k – number of components in the mixture (natural gas).

The density of natural gas cm is equal to

							at 0.1 MPa and 293 K
							at 0.1 MPa and 293 K

i is the density of the i-th component at 0.1 MPa and 293 K.

Data on individual components are shown in Table 1.

Conversion of density at different conditions temperature and pressure

0.1013 MPa (101.325 kPa) in Appendix B.

1.2. Relative gas density

In the practice of engineering calculations, the concept of relative

nary density equal to the ratio of gas density to air density at identical values pressure and temperature. Normal or normal values ​​are usually taken as reference standard conditions, while the air density is

responsibly amounts to 0 1.293 kg / m 3 and 20 1.205 kg / m 3. Then the relative

The natural gas density is equal to

1.3. Density of natural gas at pressures and temperatures

Gas density for conditions in the productive formation, wellbore, gas

wires and apparatus at appropriate pressures and temperatures determine

is calculated according to the following formula

where P and T are the pressure and temperature at the place where the gas density is calculated; 293 K and 0.1013 MPa are standard conditions when located cm;

z ,z 0 – gas supercompressibility coefficients, respectively, at Р and Т and stan-

dart conditions (value z 0 = 1).

The simplest way to determine the supercompressibility coefficient z is the graphical method. The dependence of z on the given parameters is pre-

shown in Fig. 1.

For a one-component gas (pure gas), the given parameters are determined

divided according to formulas

	and T c are critical gas parameters.
For multicomponent (natural) gases, pre-calculate
xia pseudocritical pressures and temperatures according to the dependences
	T nskn iT ci /100,
	and T c are the critical parameters of the i-th gas component.
Since the composition of natural gas is determined to butane C4 H10											or hexane C6 H14

inclusive, and all other components are combined into a remainder (pseudocom-

component) C5+ or C7+, in this case the critical parameters are determined by the form

At 100 M from 5 240 and 700d from 5 950,

M s 5 – molecular weight of C5+ (C7+) kg/kmol;

d c 5 – density of the pseudocomponent C5+ (C7+), kg/m3.

Dependence between M and		found by Craig's formula

Table 1

Indicators of natural gas components

Indicators

Components

Molecular mass,

M kg/kmol

Density, kg/m3 0.1

Density, kg/m3 0.1

Relative density

Critical volume

dm3/kmol

Critical pressure,

Critical temperature

Critical compressibility

bridge, zcr

Acentric factor

Figure 1 – Dependence of the supercompressibility coefficient z on the given parameters Ppr and Tpr 2. Laboratory methods for determining the density of natural gas 2.1. Pycnometric method The pycnometric method is established by the GOST 17310-2002 standard, in accordance with according to which the density (relative density) of gases and gas mixtures is determined. The essence of the method is to weigh a glass pycnometer with a volume of 100-200 cm3 in series with dried air and dried waste. the following gas at the same temperature and pressure. The density of dry air is a reference value. Knowing the internal volume of the pycnometer, it is possible to determine the density of natural gas of unknown composition (test gas). To do this, the internal volume of the pycnometer (“water number”) is first determined by alternately weighing the pycnometer with dried air and distilled water, the densities of which are known. Then weigh A pycnometer filled with the test gas is sewn. The difference in mass between the pycnometer with the test gas and the pycnometer with air, divided by the volume of the pycnometer (“water number”) is added to the density value of dry air, which ultimately amounts to the density of the gas under study. The output of the calculation formulas is shown below. 2.1.1. Calculation formulas The density of natural gas is determined using the pycnometric method based on the following relationships: g – gas density under measurement conditions, g/dm3 kg; vz – air density under measurement conditions, g/dm3 kg; Mg – mass of gas in a pycnometer, g; Mvs – mass of air in the pycnometer, g;

Gas - comparison of the relative molecular or molar mass of one gas with that of another gas. As a rule, it is defined in relation to the lightest gas - hydrogen. Gases are also often compared to air.

In order to show which gas is selected for comparison, an index is added before the relative density symbol of the test gas, and the name itself is written in parentheses. For example, DH2(SO2). This means that the density was calculated using hydrogen. This is read as “density of sulfur oxide over hydrogen.”

To calculate the gas density for hydrogen, you need to use periodic table determine the molar masses of the gas and hydrogen under study. If it is chlorine and hydrogen, then the indicators will look like this: M(Cl2) = 71 g/mol and M(H2) = 2 g/mol. If the density of hydrogen is divided by the density of chlorine (71:2), the result is 35.5. That is, chlorine is 35.5 times heavier than hydrogen.

The relative density of a gas does not depend in any way on external conditions. This is explained by the universal laws of the state of gases, which boil down to the fact that changes in temperature and pressure do not lead to a change in their volume. For any changes in these indicators, measurements are made exactly the same.

To determine the density of a gas experimentally, you will need a flask into which it can be placed. The flask with gas must be weighed twice: the first time - by pumping out all the air from it; the second - filling it with the gas under study. It is also necessary to measure the volume of the flask in advance.

First you need to calculate the mass difference and divide it by the volume of the flask. The result will be the gas density according to given conditions. Using the equation of state, you can calculate the desired indicator under normal or ideal conditions.

You can find out the density of some gases using a summary table, which contains ready-made information. If the gas is included in the table, then you can take this information without any additional calculations or use of formulas. For example, the vapor density of water can be found out from the table of water properties (Handbook by Rivkin S.L. et al.), its electronic analogue, or using programs such as WaterSteamPro and others.

However, for different liquids, equilibrium with vapor occurs at different densities of the latter. This is explained by the difference in the forces of intermolecular interaction. The higher it is, the faster equilibrium will occur (for example, mercury). For volatile liquids (for example, ether), equilibrium can only occur at a significant vapor density.

The density of various natural gases varies from 0.72 to 2.00 kg/m3 and higher, relative - from 0.6 to 1.5 and higher. The highest density is for gases with the highest content of heavy hydrocarbons H2S, CO2 and N2, the lowest is for dry methane gases.

Properties are determined by its composition, temperature, pressure and density. The latter indicator is determined in the laboratory. It depends on all of the above. Its density can be determined using different methods. The most accurate is weighing on precise scales in a thin-walled glass container.

More than the same indicator for natural gases. In practice, this ratio is taken as 0.6:1. Static decreases faster compared to gas. At pressures up to 100 MPa, the density of natural gas can exceed 0.35 g/cm3.

It has been established that the increase may be accompanied by an increase in the temperature of hydrate formation. Natural gas low density forms hydrates at more high temperature compared to gases with increased density.

Density meters are just beginning to be used and many questions remain related to the features of their operation and testing.

DEFINITION

Atmospheric air is a mixture of many gases. Air has a complex composition. Its main components can be divided into three groups: constant, variable and random. The former include oxygen (the oxygen content in the air is about 21% by volume), nitrogen (about 86%) and the so-called inert gases (about 1%).

Content components practically does not depend on where globe a sample of dry air was taken. The second group includes carbon dioxide (0.02 - 0.04%) and water vapor (up to 3%). The content of random components depends on local conditions: near metallurgical plants noticeable amounts of sulfur dioxide are often mixed into the air, ammonia, etc., in places where organic residues decompose. In addition to various gases, the air always contains more or less dust.

Air density is a value equal to the mass of gas in the Earth's atmosphere divided by a unit volume. It depends on pressure, temperature and humidity. Exists standard value air density - 1.225 kg/m 3, corresponding to the density of dry air at a temperature of 15 o C and a pressure of 101330 Pa.

Knowing from experience the mass of a liter of air under normal conditions (1.293 g), we can calculate the molecular weight that air would have if it were an individual gas. Since a gram molecule of any gas occupies a volume of 22.4 liters under normal conditions, the average molecular weight of air is equal to

22.4 × 1.293 = 29.

This number - 29 - should be remembered: knowing it, it is easy to calculate the density of any gas relative to air.

Density of liquid air

When sufficiently cooled, the air turns into a liquid state. Liquid air can be stored for quite a long time in vessels with double walls, from the space between which the air is pumped out to reduce heat transfer. Similar vessels are used, for example, in thermoses.

Liquid air that evaporates freely under normal conditions has a temperature of about (-190 o C). Its composition is not constant, since nitrogen evaporates more easily than oxygen. As the nitrogen is removed, the color of the liquid air changes from bluish to pale blue (the color of liquid oxygen).

In liquid air they easily transform into a solid state ethanol, diethyl ether and many gases. If, for example, carbon dioxide is passed through liquid air, it turns into white flakes similar in appearance. appearance to the snow. Mercury immersed in liquid air becomes hard and malleable.

Many substances cooled by liquid air dramatically change their properties. Thus, chink and tin become so brittle that they easily turn into powder, a lead bell makes a clear ringing sound, and a frozen rubber ball shatters if dropped on the floor.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

Exercise	Determine how many times heavier than air is hydrogen sulfide H 2 S.
Solution	The ratio of the mass of a given gas to the mass of another gas taken in the same volume, at the same temperature and the same pressure is called the relative density of the first gas to the second. This value shows how many times the first gas is heavier or lighter than the second gas. The relative molecular weight of air is taken to be 29 (taking into account the content of nitrogen, oxygen and other gases in the air). It should be noted that the concept of “relative molecular mass of air” is used conditionally, since air is a mixture of gases. D air (H 2 S) = M r (H 2 S) / M r (air); D air (H 2 S) = 34 / 29 = 1.17. M r (H 2 S) = 2 × A r (H) + A r (S) = 2 × 1 + 32 = 2 + 32 = 34.
Answer	Hydrogen sulfide H 2 S is 1.17 times heavier than air.