Moles and masses - Higher

The mass of a substance can be calculated from its amount, and the amount of a substance can be calculated from its mass. The link between the two quantities is the Mr, relative formula mass (or Ar, relative atomic mass for atoms and metals).

Calculating masses

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The mass of a given amount of substance is calculated using: {\text{mass in g}}  = M_\textup{r} \times {\text{amount~in~mol}}

One way to remember this is mass equals mister mole.

Example 1

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

mass = 44 × 0.25

= 11 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.

Example 2

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

mass = 56 × 0.10

= 5.6 g

Calculating amounts

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The amount of a given mass substance is calculated using: {\text{amout in mol}} = \frac{\text {mass in g}}{M_\textup{r}}

Again, 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