About the Dalton's Law Calculator
This Dalton’s law calculator handles gas mixtures three ways. Enter the partial pressure of each gas to get the total pressure and every gas’s mole fraction. Enter a total pressure with the moles (or percentages) of each gas to get the partial pressures. Or, for gas collected over water in the lab, enter the measured pressure and the water temperature and it subtracts the water vapour pressure to give the pressure of the dry gas.
It is built for general-chemistry students, scuba divers checking oxygen partial pressure in a blend, and anyone working with breathing gases, air composition or eudiometer experiments. Up to four gases can be included, and pressures can be in atm, kPa, mmHg (torr), bar or psi.
Dalton’s law assumes ideal-gas behaviour: each gas exerts the pressure it would have alone in the same volume and temperature. That is accurate at everyday pressures; at very high pressures intermolecular forces cause small deviations. Water vapour pressure is calculated with the Buck equation, which matches standard tables to within about 0.1% between 0 and 100 °C.
How to use the dalton's law calculator
- 1Choose what you know: partial pressures, total pressure with moles, or a gas collected over water.
- 2Pick the pressure unit you are working in.
- 3Enter each gas’s partial pressure or moles (percentages also work as moles).
- 4For gas over water, enter the measured pressure and the water temperature instead.
- 5Read the total or partial pressures and the mole fraction of each gas.
Formula and method
Dalton’s law of partial pressures states that the total pressure of a mixture of non-reacting ideal gases equals the sum of the pressures each gas would exert on its own. Because every gas obeys PV = nRT in the same volume and temperature, each partial pressure is proportional to that gas’s moles, so Pᵢ = xᵢ·P_total, where xᵢ is the mole fraction.
When a gas is collected by displacing water, the trapped gas is saturated with water vapour, so the dry-gas pressure is the measured total minus the vapour pressure of water at that temperature. The vapour pressure is computed with the Buck (1996) equation P = 0.61121·exp((18.678 − T/234.5)·T/(257.14 + T)) kPa, with T in °C.
- Pᵢ
- Partial pressure of gas i
- xᵢ
- Mole fraction of gas i (nᵢ / n_total)
- P_total
- Total pressure of the mixture
- P_H₂O(T)
- Vapour pressure of water at temperature T
Worked examples
Trimix dive gas partial pressures
Partial pressures of 0.45 atm helium, 0.30 atm oxygen and 1.25 atm nitrogen add up to 2.00 atm. Oxygen’s mole fraction is 0.30 ÷ 2.00 = 15%.
Dry air at 1 atm from mole percentages
The percentages sum to 100, so oxygen’s mole fraction is 0.2095 and its partial pressure at 1 atm is 0.2095 atm (about 159 mmHg).
Hydrogen collected over water at 25 °C
Water’s vapour pressure at 25 °C is about 23.77 mmHg, so the hydrogen itself exerts 755 − 23.77 ≈ 731.2 mmHg. Use this dry pressure in PV = nRT.
Frequently asked questions
What is Dalton's law of partial pressures?+
It states that the total pressure of a mixture of non-reacting gases equals the sum of the partial pressures of the individual gases. Each gas behaves as if it alone occupied the whole container.
How do you calculate partial pressure from mole fraction?+
Multiply the mole fraction by the total pressure: Pᵢ = xᵢ × P_total. Oxygen at 20.95% in air at 101.325 kPa has a partial pressure of 0.2095 × 101.325 ≈ 21.2 kPa.
Why subtract water vapour pressure for gas collected over water?+
Gas bubbled through water becomes saturated with water vapour, which adds its own partial pressure. Subtracting the vapour pressure at the water temperature gives the pressure of the dry gas you actually collected.
What is the vapour pressure of water at room temperature?+
About 2.34 kPa (17.5 mmHg) at 20 °C, 3.17 kPa (23.8 mmHg) at 25 °C and 4.25 kPa (31.8 mmHg) at 30 °C. It rises steeply with temperature, reaching 101.3 kPa at 100 °C.
Why does oxygen partial pressure matter for divers?+
Oxygen becomes toxic at high partial pressure. Recreational limits are typically 1.4 atm during the dive and 1.6 atm at most, so gas blends are chosen by multiplying oxygen fraction by the absolute pressure at depth.