3.5 - Reacting Mass Calculations

Reacting masses through the mole

• The numbers in front of the formulae in a balanced equation show amounts in moles. In:
  • Mg + 2HCl → MgCl₂ + H₂
• One mole of magnesium reacts with two moles of hydrochloric acid to give one mole of magnesium chloride and one mole of hydrogen.

Why you go via moles

• Two masses can't be compared directly, because a gram of one substance contains a different number of particles from a gram of another.
• The route runs: mass → amount in moles → across the equation → mass.

The method

1. Write out the balanced equation.
2. Work out the relative formula mass ($M_{\text{r}}$) of the known substance and the wanted substance.
3. Turn the known mass into an amount in moles - $\text{Moles}=\frac{\text{mass}}{M_{\text{r}}}$.
4. Use the balancing numbers in the equation to find the moles of the wanted substance.
5. Turn that amount back into a mass - $\text{Mass}=\text{moles}\times M_{\text{r}}$.

A relative formula mass is always for one unit of the formula, so the number in front plays no part in step 2 - it only applies at step 4 when comparing moles.

Worked example - Product mass through the mole

Lithium burns in air to form lithium oxide:

4Li + O₂ → 2Li₂O

Calculate the mass of lithium oxide formed when 105 g of lithium is burned in air. Relative atomic masses: O = 16, Li = 7.