Chapter 11 · Gases
11.9Mixtures of Gases
11 min · two checks
Predict
Air is about 21% O₂ by mole. If the total pressure is 100 kPa, what is the partial pressure of oxygen?
The idea
- Apply Dalton’s law of partial pressures.
- Correct a gas collected over water.
In a mixture of ideal gases, each gas behaves as if it occupied the container alone. Its partial pressure is the pressure it would exert by itself. Dalton’s law says the total pressure is the sum of the partial pressures. Because volume and temperature are shared, each partial pressure is the mole fraction times the total: P_i = (n_i / n_total) P_total. The mole percent and the pressure percent match.
Collecting a gas over water is a mixture problem in disguise. The measured pressure is gas plus water vapor. Look up the vapor pressure of water at that temperature and subtract: P_gas = P_total − P_water. Forgetting the subtraction makes the gas look like more moles than you collected. Physiology uses the same idea: oxygen’s partial pressure, not just its percent, drives how much dissolves in blood.
Keep these
- P_total = P₁ + P₂ + …
- Partial pressure = mole fraction × total pressure.
- Over water: subtract the vapor pressure of water.
Worked path
A 2.00 mol sample of gas is 0.40 mol CO₂ and the rest N₂. Total pressure is 150 kPa. Find the partial pressure of CO₂.
Observe
Mole fraction of CO₂ = 0.40/2.00 = 0.20.