Calculating the theoretical mass of a product

The theoretical mass of a product is the mass the reaction would make if all of the reactant were converted to product and nothing were lost. You can work it out from the mass of a reactant and the balanced equation.

• Step 1: moles of reactant = mass in grams ÷ relative formula mass (Mr).
• Step 2: use the balancing numbers in the balanced equation to find the moles of product.
• Step 3: mass of product = moles of product × Mr of the product.

Example: 12 g of magnesium burns in oxygen, 2Mg + O₂ → 2MgO. The Ar of Mg is 24, and the Mr of MgO is 40. Moles of Mg = 12 ÷ 24 = 0.5 mol. The equation shows 2 mol of Mg makes 2 mol of MgO, a 1 : 1 ratio, so 0.5 mol of MgO forms. Theoretical mass = 0.5 × 40 = 20 g.

Example: 50 g of calcium carbonate is heated, CaCO₃ → CaO + CO₂. The Mr of CaCO₃ is 100 and the Mr of CaO is 56. Moles of CaCO₃ = 50 ÷ 100 = 0.5 mol, so 0.5 mol of CaO forms. Theoretical mass of CaO = 0.5 × 56 = 28 g.