Avogadro's number and molar mass

Avogadro's number

One mole always corresponds to the same number of particles: Avogadro’s number, denoted by Na, which is approximately 6.02 × 10²³ per mole (also written as 6.02 × 10²³ mol⁻¹). This number is immense: if we were to count at a rate of one atom per second, it would take far longer than the age of the Universe to reach 6.02 × 10²³!

Molar mass

Molar mass, denoted by M and expressed in g/mol, is the mass of one mole of the substance in question. It is obtained by adding together the atomic masses of the elements that make up the molecule, as given in the periodic table (in g/mol, rounded: H = 1.0; C = 12.0; O = 16.0; Na = 23.0; Cl = 35.5).

Practical example: the water molecule

The water molecule H₂O contains 2 hydrogen atoms and 1 oxygen atom. Its molar mass is therefore:

M(H2O) = 2 x 1,0 + 16,0 = 18,0 g/mol

One mole of water (6.02 × 10²³ molecules) therefore weighs 18.0 grams.

Practical example: sodium chloride

Table salt (NaCl) contains 1 sodium atom and 1 chlorine atom:

M(NaCl) = 23,0 + 35,5 = 58,5 g/mol

Common mistake

Do not confuse the molar mass M (mass of one mole, in g/mol) with the mass m of a sample (in g). M is a fixed property of the substance; m depends on the actual quantity you have to hand.