Molecular mass is expressed as the sum of the masses of atoms in a 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, 1/12 of the mass of C 12 carbon atoms is taken, which in units 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 carbon atom C 12 . 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, they use the more convenient value Motn = 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 masses of other compounds are 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 air can be taken equal to 29.
Earlier in chemistry, the concept of a gram molecule was used. Today this concept is replaced by a mole - the amount of substance containing the number of particles (molecules, atoms, ions) equal to the Avogadro constant (6.022 x 10 23 ). Until today, the term "molar (molecular) weight" is also traditionally used. But, unlike weight, which depends on geographical coordinates, mass is a constant parameter, so it’s still more correct to use this concept.
The molecular mass of air, like other gases, can be found using the Avogadro law. This law states that under the same conditions, the same number of molecules are present in the same gas volumes. As a result, at a certain temperature and pressure, a mole of gas will occupy the same volume. Given that this law is strictly fulfilled for ideal gases, a mole of gas 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 a certain known volume of gas is determined at certain pressure and temperature. Then, corrections are made for the imperfection of the real gas and, using the Clapeyron equation PV = RT, the volume is reduced to pressure conditions 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 under study , that is, its molecular weight. Thus, the molecular mass of air was determined.
This method gives fairly accurate values of molecular masses, which are sometimes even used to determine the atomic masses of chemical compounds. For a rough estimate of molecular weight, a gas is usually considered ideal, and no further corrections are made.
The above method is often used to determine the molecular masses of volatile liquids.