Molecular mass of air - determination

The molecular mass is expressed by the sum of the masses of the atoms entering the molecule of matter. Usually it is expressed in amu, (atomic units of mass), sometimes also called dalton and is denoted by D. For 1 amu. Today it takes 1/12 of the mass C of ¹² carbon atoms, which in terms of mass is 1.66057.10 ^-27 kg.

Thus, the atomic mass of hydrogen, equal to 1, shows that the hydrogen atom H ¹ is 12 times lighter than the C ¹² carbon atom. Multiplying the molecular weight of the chemical compound by 1.66057.10 ^-27 , we obtain the value of the mass of the molecule in kilograms.

In practice, however, use the more convenient value Mn = M / D, where M is the mass of the molecule in the same mass units as D. The molecular mass of oxygen expressed in carbon units is 16 x 2 = 32 (the oxygen molecule is diatomic) . In the same way, the molecular weights of other compounds are also calculated in chemical calculations. The molecular mass of hydrogen, in which the molecule is also diatomic, is, respectively, 2 x 1 = 2.

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. In particular, the molecular mass of the air can be assumed equal to 29.

Previously, the concept of a gram molecule was used in chemistry. Today this concept is replaced by a mole - the amount of substance containing the number of particles (molecules, atoms, ions) equal to Avogadro's constant (6.022 x 10 ²³ ). Until now, the term "molar (molecular) weight" has also traditionally been used. But, unlike weight, which depends on geographic coordinates, mass is a constant parameter, so it is still more correct to use this concept.

The molecular mass of air, as well as of other gases, can be found with the help of Avogadro's law. This law states that under the same conditions, the same number of molecules are present in the same volumes of gases. As a consequence, at a certain temperature and pressure, the mole of gas will occupy the same volume. Given that this law is strictly satisfied for ideal gases, a gas mole containing 6.022 x 10 ²³ molecules occupies a volume of 22.414 liters at 0 ° C and a pressure of 1 atmosphere.

The molecular mass of air or any other gaseous substances is as follows. The mass of some known gas volume is determined at a certain pressure and temperature. Then, corrections are made for the nonideality of the real gas and, using the Clapeyron equation PV = RT, the volume is given to the conditions of pressure of 1 atmosphere and 0 ° C. Further, knowing the volume and mass under these conditions for an ideal gas, it is easy to calculate the mass of 22,414 liters of the gaseous substance , That is, its molecular weight. Thus, the molecular mass of the air was determined.

This method gives sufficiently accurate values of molecular masses, which are sometimes used even to determine the atomic masses of chemical compounds. For an approximate estimation of the molecular weight, the gas is usually considered ideal, and no further corrections are made.

The above method is often used to determine the molecular weights of volatile liquids.