Unit of molar mass: g/mol. Since the molecular masses of CO2 and H2O and the atomic mass of oxygen are respectively 44; 18 and 16 amu, then their molar masses are equal: a) 44 g/mol; b) 18g/mol; c) 16 g/mol.

In the same way, the molecular weights of other compounds are calculated in chemical calculations. Molecular mass is a characteristic of the average mass of a molecule; it takes into account the isotopic composition of all elements that form a given Chemical substance. This indicator can also be determined for a mixture of several substances whose composition is known. This law states that under the same conditions in the same volumes of gases there is same number molecules.

The mass of a certain known volume of gas at a certain pressure and temperature is determined. This method gives fairly accurate values ​​of molecular masses, which are sometimes even used to determine atomic masses chemical compounds. For approximate molecular weight estimates, the gas is usually assumed to be ideal and no additional corrections are made.

Air is necessary for the normal existence of living organisms on Earth. In industry and in everyday life, atmospheric oxygen is used to burn fuel to produce heat and mechanical energy in engines. internal combustion. In 1754, Joseph Black experimentally proved that air is a mixture of gases and not homogeneous substance. The first symbol is a generally accepted image of the element air. The third image is the air trigram in the Book of Changes.

Human freedom is woven from thin air. Therefore, the symbol of air is, first of all, a symbol of freedom. This is a freedom for which there are no barriers, because the air cannot be limited, it cannot be caught and shaped.

Atmospheric air is a mixture of dry air and water vapor (from 0.2% to 2.6%). Thus, the air can almost always be considered humid. The mechanical mixture of dry air and water vapor is called moist air or air-steam mixture. Absolute air humidity is the mass of vapor in 1 m3 of humid air.

When extinguishing a fire with water, both conditions are created: water cools burning objects, and its vapors make it difficult for air to reach them. The question of the composition of air in science was not immediately resolved. In 1774, the French scientist A. Lavoisier proved that air is a mixture of mainly two gases - nitrogen and oxygen. In addition, the air contains carbon monoxide (IV) and water vapor. The approximate air composition is shown in the table.

Electric welding of easily oxidized metals is carried out in the inert atmosphere of argon. Neon, argon, krypton and xenon fill light bulbs. You have already become familiar with the combustion of substances in oxygen. When substances burn in air, as a rule, the same products are formed, i.e., various oxides. The number of atoms of elements included in the composition of the burned substance is equalized: C6H6 + O2 -> 6CO2 + 3H2O3.

This technique is used when extinguishing fires in cases of burning oil and its products. Chemical composition air is important hygienic value since he plays decisive role in the implementation of the respiratory function of the body.

In residential, public and sports premises, no significant changes in oxygen content are observed, since outside air penetrates into them. With prolonged inhalation of air containing 1 - 1.5% carbon dioxide there is a deterioration in health, and at 2-2.5% pathological changes are detected.

How to find the molar mass of air

Atoms of elements are characterized by a certain (only inherent) mass. Relative molecular mass values ​​are calculated from relative atomic mass values, taking into account the number of atoms of each element in the formula unit of a complex substance. Sum mass fractions elements included in compound, is equal to 1 (100%). In chemical calculations, the mass of gaseous reactants and products is often replaced by their volume. This physical constant is molar volume gas at normal conditions.

They are based on the laws of conservation of mass, constancy of composition, multiple ratios, as well as gas laws – volumetric relations and Avogadro. In production, material balances are calculated on this basis. The law is always true for gaseous and liquid substances. The law of multiple ratios, like the law of constancy of composition, is not universal and is also not valid for substances in the solid state. For example, when 2 volumes of hydrogen and 1 volume of oxygen interact, 2 volumes of water vapor are formed. These numbers coincide with the stoichiometric coefficients in the reaction equation.

Relative atomic masses known elements are given in the table " Periodic table elements D.I. Mendeleev". The amount of substance B is a physical quantity indicating the number of formula units of the substance relative to Avogadro’s constant. Avogadro's constant, in turn, shows the number of atoms contained in 12g of the carbon isotope 126C, or the number atomic units mass in 1 g of substance.

10. Determination of molecular weights of substances in a gaseous state.

In fact, for the carbon isotope 126C Ar = 12, and the molar mass of atoms (by definition of the concept “mole”) is 12 g/mol. Under normal conditions (101.325 kPa; 273 K), the molar volume of any gas is 22.4 l/mol (more precisely, Vn = 22.4 l/mol). For not ideal gases, called real, the molar volumes are different and somewhat different from exact value. If the volume and pressure of a gas are expressed in other units of measurement, then the value of the gas constant in the Clapeyron–Mendeleev equation will take on a different value.

Therefore, they are used for illuminated advertising and in lighthouses. Combustion of substances in the air. Definition molar masses substances in a gaseous state. According to the law, equal volumes of gases taken at the same temperature and the same pressure contain equal number molecules. The molecular mass of air, like other gases, can be found using Avogadro's law.

Murom Institute (branch)

federal state budgetary educational institution

higher professional education

"Vladimir State University

named after Alexander Grigorievich and

Nikolai Grigorievich Stoletov"

Department: "FPM"

Discipline: Physics

Laboratory work No. 6.03

Approved on methodological

seminar of the department of FPM

Head department ____________

Laboratory work No. 6.03

DETERMINATION OF AIR MOLECULAR MASS

Goal of the work- get acquainted with one of the methods for determining the molecular mass of a gas and measure the molecular mass of air.

Devices and accessories: air cylinder, weighing installation or technical scales, pressure gauge, weights, vacuum pump.

SAFETY

Handle the glass container in the linen bag with care.

THEORETICAL INFORMATION

Molecular mass is the ratio of the mass of a molecule of a given substance to 1/12 the mass of a carbon atom C.

Molecular mass, according to definition, can be represented as the sum of the atomic masses of the elements that make up the molecule

 =

(1)

where A is atomic massi th element included in the composition of the molecule;

n is the number of atoms.

Methods for determining molecular weight are divided into two groups - absolute and statistical. The absolute methods that give the “true” value of molecular weight include the mass spectroscopy method. Other methods give only the average molecular weight.

Determination of the molecular mass of gases is based on the gas equation of state

PV = RT (2)

where P is gas pressure,

m, V – its mass and volume,

T – absolute temperature

R – universal gas constant,

 – statistical average molecular mass.

Equation 2 is valid only for ideal gas. An ideal gas is a gas in which there are no interaction forces (attraction or repulsion) between its molecules. Molecules of an ideal gas are represented as elastic balls of infinitesimal size. Real gases have interaction forces between molecules, and molecules cannot always be considered as elastic balls of vanishingly small sizes, therefore real gases deviate from law (2).

However, at not too high pressures, when gas molecules can freely travel long distances before colliding, the interaction of molecules can be neglected, the sizes of molecules can also be neglected (when the volume of gas is large enough), then the real gas will be close to ideal and equation (2) can be applied. At atmospheric pressure and room temperatures, many gases (nitrogen, hydrogen, helium, oxygen, air, etc.) can be considered an ideal gas with a fairly good approximation.

An important characteristic of molecular motion is the mean free path. Molecules in a gas are in a state of continuous and chaotic motion, collide with each other and freely travel a certain path between collisions . The length of this path between two collisions is different, but due to the large number of molecules and the randomness of their movement, we can talk about the average free path of the molecules. Average length the free path of molecules can be determined by the formula

=

,

where  is the effective diameter of the molecule (for air  = 0.27 10 m),

n – number of molecules per unit volume.

n =,

then taking this into account the formula , for  has the form:

 =

,

Where – molecular mass of gas, N – Avogadro's number,  – gas density.

Let the air in an open cylinder occupy the volume V, its mass T, atmospheric pressure P; pump out the air from the cylinder to P . Now the mass of air in the cylinder will be m . For these two states we write equation (2)

PV = RT (3)

P V= RT (4)

Subtracting from (3) (4), we express .

 =

=

(5)

Thus, knowing the change in mass with changes in pressure, you can find the molecular mass of air using formula (5).

INSTALLATION DESCRIPTION

A general view of the installation is shown in Fig. 1. The installation consists of a vacuum pump(1) , tap (2) (using tap 2, the system is disconnected from the vacuum pump), vacuum gauge (3). The rotation of the vacuum gauge pointer arrow is proportional to the vacuum achieved in the system, i.e. the difference between atmospheric pressure and air pressure in the installation. The zero value on the vacuum gauge scale corresponds to the atmospheric pressure in the installation. The device begins to display only when air is pumped out of the system, i.e. when the air pressure in the installation is below atmospheric.

Using tap 4, the system is connected to the atmosphere. In Fig. 1 shows a vacuum line (5), a removable cylinder (6) (volume I/1225 ml) with a rubber tube and a clamp (7), which serves to disconnect the cylinder 6 from the atmosphere when weighing the cylinder.

COMPLETING OF THE WORK

Exercise 1. Determination of molecular mass of air.

1. Open taps 2 and 4, clamp 7 and disconnect cylinder 6 from the installation. We weigh the cylinder 6 together with the rubber tube and clamp 7 and record the measurement results in Table I. Weighing must be carried out with a fairly high degree of accuracy and special attention must be paid to this operation.

2. Attach the cylinder 6 to the installation and pump out the air from the cylinder so that the change in pressure is P=0.1 . Close tap 2 and add P to the measurement table. Close tap 4, use clamp 7, disconnect cylinder 6 from the installation and weigh it. The measurement results are entered into the table.

3. Open clamp 7, smoothly open taps 2 and 4 and repeat step 2 4 more times for other values P.

4. Based on the measurement results, we calculate  air and estimate the measurement error.

Task 2. Determination of air density.

We determine the air density using the Mendeleev–Cliperon equation for ideal gases.

PV = .

From this equation it follows that, since

That

 = ,

where R = 8.31*10 J/(kmol·K), universal gas constant.

Let us first determine the air density in the cylinder before pumping, assuming that the pressure P=P equal to atmospheric pressure (P = 101 kPa). Then we determine the air density at different P taken from the measurement table, assuming that the pressure in the flask is P = P -P.

Based on the calculation results, plot the dependence of density on pressure P:

MEASUREMENT TABLE

m



Task 3. Based on the calculated calculations , determine the mean free path wavelength  and plot the dependence of  on :

.

CONTROL QUESTIONS

1. List the main provisions of the molecular kinetic theory of ideal gases.

2. What physical quantities are called gas state parameters, give their definition.

3. Formulate the laws of ideal gases.

4. Under what conditions does a gas obey the ideal gas laws?

5. Under what conditions is the Clapeyron-Mendeleev equation applicable to gases?

6. What is molecular weight, what does molecular weight depend on.

7. Air density, what does it depend on?

8. Mean free path of gas molecules and effective diameter.

9. Derive formulas for calculating  and .

10. What measurements need to be made to calculate the molecular weight  and air density .

List of used literature

1. Savelyev I.V. Well general physics. M.: Nauka, 1970. T.1, § 98.

2. A.A. Detlaff, B.M. Yavorsky. Physics course. M.: Publishing house " graduate School", 1973, pp. 175-179.

Page 1

The molecular weight of air is calculated taking into account percentage various components. The mass of an air molecule is understood as the average value of the masses of molecules contained in the air, taking into account their relative concentration.

The molecular weight of air is calculated taking into account the percentage of various components. The mass of an air molecule is the average value of the masses of the molecules contained in the air, taking into account their relative concentration.

TO; M is the molecular weight of air; rzab - air pressure at the side.

Rvzh - the same, above the surface of the evaporating liquid, kg / m3, Mw - molecular weight of air equal to 29; MP is the molecular weight of the vapor of the evaporating liquid.

So, to count the number of air molecules in earth's atmosphere, it is enough to know only the air pressure at sea level, the molecular weight of the air, the radius of the Earth and the acceleration free fall g at ​​its surface. The answer does not include the height of the atmosphere, only that it is small compared to the radius of the Earth.

Characteristics of hydrocarbon components of natural gas.| Calculation of pseudocritical temperature and pressure of natural gas.

Therefore, to determine the molecular weight of a gas, we must multiply it specific gravity(taken as 1 for air) by the molecular weight of air.

Ah, cm, in 1 sec with a pressure difference on both sides of the partition Ar, dynes / cm2; M is the molecular weight of air, g/mol; R - universal gas constant, erg/mol - deg.

Based on the laws of ideal gases, it can be shown that the specific gravity of a gas is also equal to the ratio of the molecular weight of the gas to the molecular weight of air.

However, there may also be a discrepancy between the vapor permeability of the material and the air permeability of the fence structure made of the same material. This happens due to the inevitable presence of leaks and cracks in the structure, which significantly increase the air permeability of the fence, and also because the molecular weight of air and water vapor is not the same.

To characterize natural gases, its specific gravity is widely used. Relative density gas is expressed by the ratio of the density of gas at atmospheric pressure and standard temperature to the density of air at the same pressure and temperature. Since at atmospheric pressure and certain temperature The densities of gases are directly proportional to their molecular weights; the relative density of a gas can be represented as the ratio of the molecular weight of the gas to the molecular weight of air. The relative density of natural gases varies from 0 6 to 1 1 depending on the relative concentration of heavier hydrocarbons in the gas.

In carbon dioxide plants, dry ice production also requires removing air from the system. In addition to the air infiltration paths mentioned earlier, in dry ice machines in ice makers, air is systematically introduced into the system when ice blocks are removed from the ice makers. After the ice block falls out, the volume of the ice maker is filled with air, which, when the ice maker is turned on, is sucked off by the compressor and, together with carbon dioxide, is supplied to the condenser. Due to the fact that the molecular weight of carbon dioxide is greater than the molecular weight of air, the nature of the lines of the pa / (ga) graph is similar to that for freons. But since the molecular weights of carbon dioxide and air, compared with freons, differ little from each other, the graph lines are close to straight lines over a significant area. The nomogram for carbon dioxide, constructed by R. R. Skvarchenko (VNIHI), is shown in Fig.

In carbon dioxide plants, dry ice production also requires removing air from the system. In addition to the air infiltration paths mentioned earlier, in dry ice machines in ice makers, air is systematically introduced into the system when ice blocks are removed from the ice makers. After the ice block falls out, the volume of the ice maker is filled with air, which, when the ice maker is turned on, is sucked off by the compressor and, together with carbon dioxide, is supplied to the condenser. Due to the fact that the molecular weight of carbon dioxide is greater than the molecular weight of air, the nature of the lines of the graph pa f (ga) is similar to that for freons. But since the molecular weights of carbon dioxide and air, compared with freons, differ little from each other, the graph lines are close to straight lines over a significant area.