## AS-Level Amounts of Substance

• The number of moles in a sample can be calculated using the formula:

• number of moles = mass (g) / molar mass

• The molar mass (relative molecular mass or relative formula mass) of a compound is calculated by adding up all the relative atomic masses of the atoms within the compound’s formula.

## QUICK NOTES

Calculating Number of Moles Using Mass

There are two main ways of measuring how much of a substance you have - mass and volume.

We know that atoms of different elements have different relative atomic masses (see relative atomic mass); we also know that these atomic masses refer to the mass (in grams) of one mole’s worth of that atom.

Calculating moles

Let’s take oxygen. Oxygen has a relative atomic mass of 16. This means, 1 mole of oxygen atoms has a combined mass of 16g.

This means we can say:

number of moles = mass (grams) / relative atomic mass (grams per mole)

Let’s check: if we have 16g of oxygen, how many moles do we have?

We need to know the mass (16g) and we can look-up the relative atomic mass of oxygen on the periodic table (16 g per mole).

number of moles = 16g / 16g per mole = 1 mole

There is a problem, and you may have spotted it. Oxygen does not actually exist as lone atoms – oxygen is found in elemental form (bonded to itself) as the molecule O .

O is a molecule, not an atom. So, we cannot use relative atomic mass to calculate the number of moles we have in a given mass. Instead, we need to use relative molecular mass.

Relative molecular mass is the relative atomic masses of all the atoms within a molecule added together. In one molecule of oxygen (O ) we have two atoms of oxygen. The relative atomic mass of oxygen is 16 g per mole. If we have two atoms, then the relative molecular mass will be 16 + 16 = 32 g per mole.

So, if we have 16 grams of oxygen gas. How many moles of oxygen molecules (O ) would we have?

Remember the equation:

number of moles = mass (grams) / relative molecular mass (grams per mole)

mass of O  = 16 grams

relative molecular mass = 16 + 16 = 32 grams per mole

number of moles = 16 / 32 = 0.5 moles

This means we have 0.5 moles of O molecules in 16 grams of oxygen gas. The number of oxygen atoms in 16 grams of oxygen gas would simply be 1 mole.

If we have 0.5 moles of O molecules and 1 molecule is made of 2 oxygen atoms then the number of oxygen atoms we have would be 0.5 x 2 = 1 mole.

2

2

2

2​

2​

2​

2