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

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

Example 2

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.

Question

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

Mass = 56 × 0.10

= 5.6 g

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.

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