Dalton's Law Calculator (Partial Pressures)
Calculate partial pressures, mole fractions and total pressure of a gas mixture with Dalton's law, with the working shown.
What this calculator does
Enter the total pressure and a mole fraction (or the partial pressures) and find the pressure of each gas in the mixture.
When you need it
Dalton's law is used for gases collected over water, for breathing gas mixtures in diving and medicine, and for any problem with more than one gas in a container.
Formulas
- P(total) = P₁ + P₂ + P₃ + …
- Pᵢ = χᵢ · P(total)
- χᵢ = nᵢ ÷ n(total)
How to use it
- Choose to find a partial pressure, the total pressure or a mole fraction.
- Enter the known pressures or mole fraction.
- Read the result and the mole fraction formula.
Worked example
A mixture of 0.50 mol He and 1.50 mol Ne is at a total pressure of 2.00 atm. Find the partial pressure of each.
- n(total) = 2.00 mol
- χ(He) = 0.50 ÷ 2.00 = 0.25, so P(He) = 0.25 × 2.00 = 0.50 atm
- χ(Ne) = 0.75, so P(Ne) = 0.75 × 2.00 = 1.50 atm
Answer
P(He) = 0.50 atm and P(Ne) = 1.50 atm, which add up to 2.00 atm.
Common mistakes
- Using masses instead of moles for mole fractions.
- Forgetting to subtract the water vapor pressure for a gas collected over water.
- Letting the mole fractions add up to something other than 1.
Related tools
Frequently asked questions
What is Dalton's law?
The total pressure of a mixture of gases is the sum of the pressures each gas would exert alone.
What is a partial pressure?
The pressure a single gas in a mixture would exert if it occupied the container alone.
How do you find partial pressure from mole fraction?
Multiply the mole fraction by the total pressure.