Last updated: June 2026
Use this partial pressure and mole fraction calculator to work with gas mixtures and Dalton’s law. It can calculate partial pressure, mole fraction or total pressure, and it also includes a separate mole-fraction-from-moles calculator.
- Choose the quantity you want to calculate.
- Enter the two known quantities, or select a preset to fill both fields.
- Select the pressure unit used in the question.
- Press Calculate to see the substitution and answer.
- Use the separate mole-fraction calculator when the question gives amounts in moles.
What is partial pressure?
Partial pressure is the pressure contributed by one gas in a mixture. It is the pressure that gas would exert if it alone occupied the container at the same temperature and volume.
Dalton’s law of partial pressures
Dalton’s law states that the total pressure of a gas mixture equals the sum of the partial pressures of the gases present.
What is mole fraction?
Mole fraction is the fraction of the total amount of gas contributed by one component. It has no unit because it is a ratio of moles.
The mole fractions of all gases in a mixture add up to 1.
Connecting mole fraction and partial pressure
A gas with a mole fraction of 0.25 contributes 25% of the total pressure.
Worked examples
Example 1: Finding partial pressure
If xO₂ = 0.250 and Ptotal = 120 kPa:
Example 2: Finding mole fraction
If pO₂ = 30.0 kPa and Ptotal = 120 kPa:
Example 3: Finding mole fraction from moles
A mixture contains 0.200 mol of oxygen and 1.000 mol of gas in total.
Common exam traps
Frequently asked questions
Can I use atm instead of kPa?
Yes. You may use atm, kPa, Pa, bar or torr as long as the same pressure unit is used consistently.
Can mole fraction be greater than 1?
No. Mole fraction represents a fraction of the total amount, so it must be between 0 and 1.
Why are mole fraction and partial pressure taught together?
They are directly connected through pi = xiPtotal, so mole fraction is often the first step in a partial-pressure calculation.