Gas Stoichiometry & Molar Volume Calculator

Gas Stoichiometry Guide

The Molar Gas Volume

At a given temperature and pressure, one mole of any ideal gas occupies the same volume regardless of its identity. At RTP (25°C, 1 atm): 24.0 dm³/mol (24,000 cm³/mol). At STP (0°C, 1 atm): 22.4 dm³/mol. The molar gas volume simplifies gas calculations: instead of PV=nRT, just multiply moles by the molar volume (or divide volume by it). This is why temperature and pressure conditions must be stated clearly in any gas volume calculation.

Gas Stoichiometry Steps

Step 1: write the balanced equation. Step 2: find moles of known substance (mass / molar mass, or use volume if a gas). Step 3: use the molar ratio from the equation to find moles of desired gas. Step 4: multiply by molar volume to find volume. Example: CaCO₃ → CaO + CO₂. Mass of CaCO₃ = 10g, Mr = 100. Moles CaCO₃ = 0.1 mol. Ratio 1:1, so moles CO₂ = 0.1 mol. Volume at RTP = 0.1 × 24 = 2.4 dm³.

Volume Ratios in Gas Reactions

When all reactants and products are gases at the same temperature and pressure, volumes are in the same ratio as the molar ratio in the balanced equation (Avogadro's law). H₂ + Cl₂ → 2HCl: 1 volume H₂ reacts with 1 volume Cl₂ to produce 2 volumes HCl. N₂ + 3H₂ → 2NH₃: 1 volume N₂ + 3 volumes H₂ → 2 volumes NH₃ (note: 4 volumes of reactants produce only 2 volumes of product — important for the Haber process equilibrium).

Real Gas vs Ideal Gas Behaviour

At RTP, most gases behave approximately ideally. Deviations occur at high pressure (molecules are close together — intermolecular forces become significant) and low temperature (molecules move slowly — attractions matter more). The van der Waals equation corrects for these: (P + an²/V²)(V − nb) = nRT. For A-level calculations, ideal gas behaviour is assumed. Real gases that deviate most from ideal: NH₃, SO₂, CO₂ (strong intermolecular attractions). Noble gases deviate least (minimal intermolecul