Amounts can be moles, mole percent or volume percent — anything proportional to moles.
| Gas | Mole fraction | Partial pressure |
|---|---|---|
| N2 | 0.7808 | 0.7808 atm |
| O2 | 0.2095 | 0.2095 atm |
| Ar | 0.0093 | 0.0093 atm |
| CO2 | 0.0004 | 4.000 × 10⁻⁴ atm |
Dalton's law: pᵢ = xᵢ × P_total, where xᵢ = nᵢ ÷ Σn. The partial pressures add up to the total pressure.
How it works
Dalton's law states that each gas in a mixture exerts the pressure it would have on its own, and the partial pressures add up to the total. The partial pressure of gas i is its mole fraction times the total pressure: pᵢ = xᵢ × P_total. Amounts can be moles, mole percent or volume percent, because for ideal gases all three are proportional.
The default composition is dry air (78.08 % N₂, 20.95 % O₂, 0.93 % Ar, about 0.04 % CO₂). When a gas is collected over water, subtract the vapour pressure of water at that temperature from the total to get the pressure of the dry gas.
Frequently asked questions
- What is the partial pressure of oxygen in air?
- About 0.2095 × 1 atm ≈ 0.21 atm (21.2 kPa) at sea level. At the top of Mount Everest, where the total pressure is about a third of that, the oxygen partial pressure is also about a third.
- How do I find a mole fraction?
- Divide the moles of that gas by the total moles of all gases. Mole fractions always add up to 1.
- Does Dalton's law work for real gases?
- Very well at ordinary pressures. At high pressures, attractions and molecular volumes make small deviations.
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