Moles and masses - Higher

The mass of a substance can be calculated from the number of moles, and the number of moles of a substance can be calculated from its mass. The link between the two quantities is the relative formula mass.

Calculating masses

The mass of a given amount of substance is calculated using:

mass = relative atomic or formula mass × amount

Worked example

Calculate the mass of 0.25 mol of carbon dioxide molecules. (Mr of CO2 = 44)

Mass = relative formula mass × amount

= 44 × 0.25

= 11 g

Worked example

Calculate the mass of 0.10 mol of iron. (Ar of Fe = 56)

Mass = 56 × 0.10

= 5.6 g

The calculation is the same if a substance is a metal or exists as separate atoms, but its Ar is used instead of an Mr.

Calculating amounts in moles

curriculum-key-fact
The amount of a given mass substance is calculated using: amount=\frac{mass}{relative~atomic~or~formula~mass}

Use Ar instead of Mr for metals or separate atoms.

Worked example

Calculate the amount of carbon atoms in 6.0 g of carbon. (Ar of C = 12)

Amount = \frac{\textup{mass}}{A_{\textup{r}}}

Amount = \frac{\textup{6.0}}{\textup{12}}

= 0.5 mol

Question

Calculate the amount of water molecules in 36 g of water. (Mr of water = 18)

Amount = \frac{mass}{relative~formula~mass}

Amount = \frac{\textup{36}}{\textup{18}}

= 2.0 mol