Chemistry · General chemistry I · Worked example
Find partial pressures with Dalton’s law
A gas mixture contains 0.300 mol of N₂ and 0.100 mol of O₂ at a total pressure of 2.00 atm. What is the partial pressure of each gas?
Find the mole fractions
The mixture holds 0.400 mol in all. Nitrogen is 0.300/0.400 = 0.750 of the moles, and oxygen is 0.100/0.400 = 0.250.
Multiply by the total pressure
Each gas contributes its mole fraction of the total pressure.
Check that they add up
For oxygen, 0.250 × 2.00 atm = 0.500 atm, and 1.50 atm + 0.500 atm = 2.00 atm, the total.
Result
The partial pressure of N₂ is 1.50 atm and that of O₂ is 0.500 atm.
Your turn
Hydrogen is collected over water at 25 °C and a total pressure of 755 mmHg. The vapor pressure of water at 25 °C is 23.8 mmHg. What is the partial pressure of the hydrogen?
Show the answer and explanation
731 mmHg.
The total is the sum of the two partial pressures, so P(H₂) = 755 − 23.8 = 731.2 mmHg, which rounds to 731 mmHg because 755 has no decimal places.
Keep exploring
Open the mixture in Gas laws & mixtures and add 0.100 mol of argon at the same total pressure: every mole fraction, and so every partial pressure, changes.
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