Partial pressure calculator (Dalton’s law)
List the gases and their amounts — moles, mole percent or any numbers in the right proportion — and the total pressure. The calculator gives each gas’s mole fraction and partial pressure. In the second mode, enter partial pressures to get the total pressure and the share of each gas.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose whether your numbers are amounts (to find partial pressures) or partial pressures (to find the total).
- Optionally type the gas names, in the same order as the numbers.
- Enter the numbers, separated by spaces or commas.
- For amounts, enter the total pressure and pick the unit.
- Mole fraction: xᵢ = nᵢ ÷ (n₁ + n₂ + …)
- Partial pressure: Pᵢ = xᵢ × P(total)
- Dalton’s law: P(total) = P₁ + P₂ + …
Mole fractions always add up to 1, and the partial pressures to the total pressure.
Worked example
Air at 1 atm, rounded: N₂ 78, O₂ 21, Ar 1 (mole percent, sum 100).
x(O₂) = 21 ÷ 100 = 0.21, so P(O₂) = 0.21 × 1 atm = 0.21 atm. Nitrogen: 0.78 atm, argon 0.01 atm. Together: 1 atm.
The same numbers work as moles: 78 mol, 21 mol and 1 mol give the same fractions.
A mixture with partial pressures 30, 50 and 20 kPa has a total of 100 kPa, and the second gas makes up half of it (x = 0.5).
Tips
- Same unit for everything. Amounts can be in any unit, but all in the same one.
- Ideal gases. Dalton’s law holds well at ordinary pressures; at very high pressures real gases deviate.
- Diving and altitude. The oxygen partial pressure, not the percentage, is what the body responds to: 21 % at a total of 0.5 atm is only about 0.1 atm of O₂.
Frequently asked questions
What is Dalton’s law of partial pressures?
In a mixture of gases that do not react, the total pressure is the sum of the pressures each gas would exert alone in the same volume: P = P₁ + P₂ + … Each partial pressure is the gas’s mole fraction times the total: Pᵢ = xᵢ × P.
How do I get the mole fraction?
Divide the moles of one gas by the moles of all gases together: xᵢ = nᵢ ÷ Σn. Mole percent divided by 100 is the same number, so you can enter composition in percent and get the same result.
What are the partial pressures in air?
Dry air is about 78.08 % N₂, 20.95 % O₂, 0.93 % Ar and 0.04 % CO₂ by moles. At 1 atm that gives 0.7808 atm of nitrogen and 0.2095 atm of oxygen. The default example rounds this to 78, 21 and 1.
Can I enter masses in grams?
Not directly: partial pressure follows moles, not mass. Convert each mass to moles first by dividing by the molar mass (32 g/mol for O₂, 28 g/mol for N₂).
Does it work for water vapour over a liquid?
Yes. A gas collected over water is a mixture with water vapour, so its pressure is the total minus the vapour pressure of water at that temperature. Use the second mode to check totals, or subtract the vapour pressure from your measured total.