Molecular weight: the essence of this chemical indicator, methods for determining

Molecular mass is the weight of all atoms in the corresponding molecule. This indicator is expressed in amu. (atomic mass units).

This chemical value is sometimes calculated in kilograms. To do this, it is multiplied by 1,66057 * 10^-27. This number is a mass of 1 amu. and corresponds to 1/12 of the weight of one nuclide atom ¹²C. Most often use the relative molecular mass, which is the ratio of the weight of the molecule to the mass of one atomic unit.

It should be noted that this indicator helps to characterize the mass of the molecule, taking into account all the isotopes that form the corresponding chemical compound.

In the macroscopic unit of the quantity of a substance, which is called "mole", there is a constant number of any particles - atoms or molecules, electrons or pairs of ions. This amount is 6.022 * 10²³ (the so-called Avogadro constant).To obtain the relative molecular weight, multiply the weight of molecules by their number and divide the result by the product of Avogadro constant and the mass of one atomic unit.

How is the molecular weight of a substance determined?

The simplest way to determine this parameter is to use a special formula, according to which the weight of the compound should be divided by its quantity.

If the chemical formula of the compound is known,The molecular weight is easily determined using the periodic table, since in it this value is indicated for each element and corresponds to its ordinal number. It suffices to find the sum of the molecular masses of all the components of the compound.

According to Avogadro's law, in gases of equal volumecontains the same number of molecular particles. Given this pattern, the Mendeleev-Klaperon equation was derived. The molecular mass of a gas can be determined using the appropriate formula.

To perform the calculations, it is necessary to know quite a few indicators - the weight of the substance, the universal gas constant, the temperature in Kelvin, as well as the pressure in Pascals and the volume in m³. Due to this amount of data, the errors in the calculations are minimal, but it should be noted that the Mendeleev-Clayperon equation is valid only for gaseous substances.

Molecular mass of volatile compounds candetermined by mass spectrometry or gas chromatography using Martin gas weights. The average value of this indicator for polymeric substances with a high degree of polymerization is calculated taking into account their rheological characteristics, as well as the properties of their solutions (for example, viscosity or ability to diffuse light).

What determines the molecular weight?

The numerical value of this quantity is used foridentification of a variety of chemical compounds, as well as to identify individual nucleotides in the test substance. In addition, this chemical indicator is often determined in the study and synthesis of high-molecular compounds, the properties of which depend precisely on this parameter.