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

The molecular weight is the weight of all atoms entering the corresponding molecule. This indicator is expressed in amu. (Atomic mass units).

This chemical value is sometimes calculated in kilograms. To do this, multiply by 1.66057 * 10 ^-27 . This number is a mass of 1 amu. And corresponds to 1/12 of the weight of one atom of the nuclide ¹² C. The most commonly used is the relative molecular mass, which is the ratio of the weight of the molecule to the mass of one atomic unit.

It is worth noting that this indicator helps to characterize the mass of a molecule taking into account all the isotopes that form the corresponding chemical compound.

In a macroscopic unit of the quantity of matter called "mole", there is a constant number of any particles - atoms or molecules, electrons or ion pairs. This number is 6,022 * 10 ²³ (the so-called constant Avogadro). To obtain the relative molecular mass, the weight of the molecules should be multiplied by their number and the resulting result divided by the product of the Avogadro constant and the mass of one atomic unit.

How is the molecular weight of the substance determined?

The simplest way to determine this parameter is to use a special formula, according to which the weight of the connection is divided into its number.

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

According to Avogadro's law, gases of the same volume contain the same number of molecular particles. Taking this into account, the Mendeleev-Klaiperon equation was derived. The molecular weight of the gas can be determined using the appropriate formula. In order to perform the calculations, it is necessary to know a sufficient number of parameters-the weight of the substance, the universal gas constant, the temperature in the Kelvin, and the pressure in the Pascals and the volume in m ³ . Due to this amount of data, the errors in the calculations are minimal, but it should be noted that the Mendeleev-Klaiperon equation is valid only for gaseous substances.

Molecular masses of volatile compounds can be determined by mass spectrometry or gas chromatography using Martin gas weights. The average value of this indicator for polymer 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 the ability to scatter light).

What is the molecular weight for?

The numerical value of this value is used to identify 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 whose properties depend precisely on this parameter.