You can use a balanced equation to calculate the mass of a product or reactant when you are given the mass of just one substance. Reacting masses are in the same ratio as the relative formula masses multiplied by the balancing numbers in the equation.
- Write the balanced equation.
- Work out the relative formula mass of the substance you know and the substance you want, and multiply each by its balancing number.
- This gives the masses in the equation. Scale them by dividing and multiplying to match the mass you have been given.
Example: 2Mg + O2 → 2MgO. The relative formula mass of MgO is 40. So 2 × 24 = 48 g of magnesium makes 2 × 40 = 80 g of magnesium oxide. If you start with 6 g of magnesium, you divide by 48 and multiply by 80: 6 ÷ 48 × 80 = 10 g of magnesium oxide. Likewise 12 g of magnesium makes 20 g.
Example: CaCO3 → CaO + CO2. The relative formula mass of CaCO3 is 100 and of CaO is 56, so 100 g makes 56 g of calcium oxide. 25 g is a quarter of 100 g, so it makes a quarter of 56 g, which is 14 g.