Calculating masses from balanced equations

A balanced equation lets you calculate the mass of a reactant needed or a product made. The numbers in front of the formulae give the mole ratio, so the method always follows the same steps. First write the balanced equation. Next convert the known mass to moles using moles = mass ÷ M_r. Then use the equation ratio to find the moles of the substance you want. Finally convert to a mass using mass = moles × M_r.

Example one: what mass of magnesium oxide forms from 12 g of magnesium? The equation is 2Mg + O₂ → 2MgO. The amount of magnesium is 12 ÷ 24 = 0.50 mol. The ratio of Mg to MgO is 2 : 2, so 0.50 mol of Mg makes 0.50 mol of MgO. With M_r = 40, the mass is 0.50 × 40 = 20 g.

Example two: 50 g of calcium carbonate is heated. The equation is CaCO₃ → CaO + CO₂. The M_r of CaCO₃ is 100, so the amount is 50 ÷ 100 = 0.50 mol. The ratio is 1 : 1, so 0.50 mol of carbon dioxide forms. With M_r = 44, the mass of carbon dioxide is 0.50 × 44 = 22 g. Check that your answer is sensible: the product masses should add up to the reactant mass when everything is counted.