Partial Pressures
Chemistry: IB Diploma Higher Level, topic 5.7.3
Partial Pressures of Gases
If you have a mixture of gases, you can split the total pressure into the pressures of each individual gas.
Partial pressures
- Say you have a 50/50 mix of water vapor and nitrogen gas at a pressure of 1 bar.
- You can split this into a partial pressure for water vapor and nitrogen gas.
- This follows from the ideal gas law. This states that every gas occupies the same volume under the same conditions.
- We would say that the water vapor has a partial pressure of 0.5 bar and the nitrogen gas has a partial pressure of 0.5 bar.
A different mixture
- If we had a 75/25 mixture of oxygen and argon at a pressure of 1 bar, we would say that:
- The partial pressure of oxygen is 0.75 bar.
- The partial pressure of argon is 0.25 bar.
Mole fractions
- Another way to think about this is using mole fractions.
- If you have 3 moles of oxygen gas and 2 moles of nitrogen gas, you can define a mole fraction of oxygen and nitrogen.
- The mole fraction of oxygen is ⅗.
- The mole fraction of nitrogen is ⅖.
- If you multiply the mole fraction by the total pressure, you get the partial pressures of the gases.