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What Is Molar Mass? Definition and Examples

What is molar mass? Definition and examples

Molar mass is the ratio of the amount of substance to its mass. Numerically, the molar mass is equal to the mass of one mole of a substance, that is, the mass of a substance that contains a number of particles equal to Avogadro's number.

The molar mass of a chemical compound is the sum of the atomic masses of the elements that compose it, multiplied by the stoichiometric coefficients of the elements by the chemical formula of the compound.

The atomic mass of an element is a value that can be found on the periodic table of the elements.

Strictly speaking, the mass of a molecule is less than the mass of its component atoms by an amount equal to the binding energy of the molecule. However, this mass variation is between 9 and 10 times smaller than the mass of the molecule, and can be neglected.

What Is the Difference Between Molar Mass and Molecular Mass?

The molar mass, expressed in g / mol, is numerically the same as the molecular mass, expressed in atomic mass units (uo Dalton), and relative molecular weight.

However, it is necessary to clearly understand the difference between molar mass and molecular mass, bearing in mind that they are the same only numerically and differ in dimension.

In What Units Is Molar Mass Measured?

In the International System of Units (SI), the unit for measuring molar mass is kilogram per mole (kg / mole). However, when the molar mass is expressed in g / mol, its numerical value is the same with the relative molecular weight. For this reason, historically molar weight is generally expressed in grams per mole (g / mole).

Molar mass in formulas is usually indicated by a capital letter M.

Direct comparisons and measurements of the masses of atoms and molecules are made using mass spectrometric methods.

Examples of Molar Mass

For example, the molar mass of oxygen as an element M (O) = g / mol, and in the form of a simple substance consisting of O 2 molecules , M (O 2 ) = 32 g / mol.

Another example: the atomic mass of iron is 55,847 a. E. m. Therefore, one mole of iron (that is, the number of iron atoms equal to Avogadro's number, ≈6.022⋅1023) has a mass of 55.847 g.

To calculate the number of the molar masses of complex molecules, it can be determined by adding the molar masses of their constituent elements. For example, the molar mass of water H 2 O is

M (H2O) = 2 M (H) + M (O) = 2.1 g / mol + 16 g / mol = 18.1 g / mol


Published: August 22, 2021
Last review: August 22, 2021