Chapter 11 · Gases
11.10Gases in Chemical Reactions
12 min · two checks
Predict
At STP, what volume of O₂ reacts with 1.00 mol of H₂ in 2 H₂ + O₂ → 2 H₂O?
The idea
- Convert gas volume at STP to moles and into a stoichiometry path.
- Use PV = nRT when conditions are not STP.
Gas stoichiometry is ordinary stoichiometry with one extra door into moles. At STP, liters ÷ 22.4 L/mol = moles. Away from STP, n = PV/RT. After that, the coefficient ratio and the molar masses do what they did in Chapter 8. Products that are gases can leave the map as liters the same way.
Watch the substance. The molar volume converts liters of a gas to moles of that gas, not to grams of a solid product, until you take the mole-ratio step. And 22.4 is not R. Students sometimes divide by 22.4 at 25 °C. If the problem is not at 0 °C and 1 atm, use the ideal gas law.
Keep these
- STP shortcut: 22.4 L = 1 mol of ideal gas.
- Otherwise n = PV/RT, then use the mole ratio.
- 22.4 L/mol is not valid at room temperature.
Worked path
How many liters of CO₂, at STP, form when 4.00 g of CH₄ burn? CH₄ + 2 O₂ → CO₂ + 2 H₂O. Molar mass CH₄ = 16.04 g/mol.
Observe
4.00/16.04 = 0.249 mol CH₄. Ratio of CO₂ to CH₄ is 1:1.