Molar Volume Calculation - 2

Many reactions produce a gas, and the molar volume lets you predict the volume of gas from the mass of a reactant. The method always has three steps:1. Find the amount of the known substance: amount = mass ÷ M_r2. Use the mole ratio from the balanced equation to find the amount of gas.3. Convert to volume: volume (dm³) = amount × 24.0 at r.t.p.Worked example 1: 2.43 g of magnesium reacts with excess hydrochloric acid. What volume of hydrogen forms at r.t.p.?Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)amount of Mg = 2.43 ÷ 24.3 = 0.100 molratio Mg : H₂ = 1 : 1, so amount of H₂ = 0.100 molvolume of H₂ = 0.100 × 24.0 = 2.40 dm³Worked example 2: 5.00 g of calcium carbonate is heated until it fully decomposes. What volume of carbon dioxide forms at r.t.p.?CaCO₃(s) → CaO(s) + CO₂(g)amount of CaCO₃ = 5.00 ÷ 100.1 = 0.0500 mol, so 0.0500 mol of CO₂ formsvolume of CO₂ = 0.0500 × 24.0 = 1.20 dm³The calculation also works backwards: divide a measured gas volume by 24.0 to get moles, use the ratio, then multiply by M_r to find the mass of reactant used.