Introduction

The subscripts in the formula for a substance indicate both

the relative numbers of the atoms in one formula unit of the substance, and
the relative numbers of moles of those atoms in one mole of the substance.

The molar mass (M) is the mass of one mole of a substance and has the unit g mol^–1.

The molar mass for a substance can be calculated from its formula and the molar masses of the atoms comprising the substance. The relevant molar masses are accessed through links in the pages of the following module.

The molar mass of methanol CH₃OH can be calculated from the molar masses of C, H and O.
M(CH₃OH) = (12.01 + 4 × 1 + 16 ) g mol^–1 = 32 g mol^–1

The amount in moles of substance in a given mass in grams can be calculated by dividing the mass by the molar mass.

If the molar mass of CH₃OH is 32 g mol^–1 ,
3.2 g CH₃OH is 0.10 mole CH₃OH.

The number of molecules (or formula units) in a given amount in moles of the molecule equals that amount in moles multiplied by the Avogadro constant.

number of molecules (formula units) = n ×

number of CH₃OH molecules = 0.10 mol × 6.02 × 10²³ mol^–1
number of CH₃OH molecules = 6.02 × 10²²

The amount in moles of an atom in a given amount in moles of molecules (formula units) containing that atom is equal to the number of that type of atom present multiplied by the given amount in moles.

In CH₃OH, there are four hydrogen atoms.
The amount in moles of H atoms in 0.10 mol CH₃OH is 0.40 mol

The number of atoms in a given amount in moles of atoms containing that atom is equal to the amount in moles multiplied by the Avogadro constant.

Each mole of H atoms has 6.02 × 10²³ atoms
The calculation is done below for the 0.40 mol of H atoms in 0.10 mol CH₃OH
0.4 mol H atoms × 6.02 × 10²³ atoms mol^–1 = 2.4 × 10²³H atoms