Gas stoichiometry uses a balanced chemical equation to predict how much gas is produced or consumed in a reaction. It matters because many reactions in labs, engines, airbags, and industrial processes involve gases whose volumes can be measured directly. The key idea is that coefficients in a balanced equation compare moles, not grams or liters directly.
Once moles are known, gas laws connect those moles to volume.
Key Facts
- Balanced equation coefficients give mole ratios, such as 2 mol H2 : 1 mol O2 : 2 mol H2O.
- At STP, 1 mol of an ideal gas occupies 22.4 L.
- Ideal gas law: PV = nRT.
- To convert gas volume at STP to moles: n = V / 22.4 L.
- To convert moles to gas volume at STP: V = n × 22.4 L.
- General gas stoichiometry path: given amount → moles → mole ratio → moles of target gas → volume.
Vocabulary
- Gas stoichiometry
- Gas stoichiometry is the use of balanced chemical equations and gas laws to calculate amounts of gases in reactions.
- Mole ratio
- A mole ratio is a conversion factor made from the coefficients of a balanced chemical equation.
- STP
- STP means standard temperature and pressure, commonly taken as 0°C and 1 atm for gas stoichiometry.
- Molar volume
- Molar volume is the volume occupied by 1 mole of a gas, equal to 22.4 L for an ideal gas at STP.
- Ideal gas law
- The ideal gas law, PV = nRT, relates pressure, volume, moles, and temperature for an ideal gas.
Common Mistakes to Avoid
- Using liters directly in the balanced equation is wrong because coefficients compare moles, not volumes unless gases are at the same temperature and pressure.
- Forgetting to balance the equation first gives incorrect mole ratios and makes every later calculation unreliable.
- Using 22.4 L per mole at non-STP conditions is wrong because molar volume changes with temperature and pressure.
- Mixing units in PV = nRT is wrong because pressure, volume, temperature, and R must use matching units, such as atm, L, K, and 0.0821 L·atm/(mol·K).
Practice Questions
- 1 At STP, how many liters of O2 are needed to completely react with 4.00 mol H2 in the reaction 2H2 + O2 → 2H2O?
- 2 Calcium carbonate decomposes by CaCO3 → CaO + CO2. How many liters of CO2 are produced at STP from 0.750 mol CaCO3?
- 3 A reaction produces the same number of moles of two different gases in separate containers. Explain why their volumes are equal at the same temperature and pressure, even if their molar masses are different.