Calculating reacting masses from equations

A balanced equation tells you the ratio of moles, so you can use it to find the mass of a reactant or product. Follow four steps. First write the balanced equation. Then work out the moles of the substance you know, using moles = mass ÷ M_r. Next use the ratio in the equation to find the moles of the substance you want. Finally convert to mass, using mass = moles × M_r.

Example: what mass of magnesium oxide forms from 6 g of magnesium? The equation is 2Mg + O₂ → 2MgO. Moles of Mg = 6 ÷ 24 = 0.25 mol. The ratio of Mg to MgO is 1 : 1, so 0.25 mol of MgO forms. Mass of MgO = 0.25 × 40 = 10 g. The oxygen needed is 10 − 6 = 4 g, because mass is conserved.

Example: heating 50 g of calcium carbonate, CaCO₃ → CaO + CO₂. Moles of CaCO₃ = 50 ÷ 100 = 0.5 mol. The ratio of CaCO₃ to CaO is 1 : 1, so 0.5 mol of CaO forms. Mass of CaO = 0.5 × 56 = 28 g. Always use the M_r of the formula as written, and check the units.