What is 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 each component gas: P_total = P_1 + P_2 + ... + P_n. Each component's partial pressure equals its mole fraction times the total pressure: P_i = x_i × P_total.

What is a mole fraction?

A mole fraction (x_i) is the ratio of the number of moles of a component to the total number of moles in the mixture. Mole fractions are dimensionless and all mole fractions in a mixture must sum to exactly 1 (or 100% if entered as percentages).

What is the partial pressure of oxygen in air?

Dry air is approximately 20.95% oxygen by mole fraction (x = 0.2095). At standard atmospheric pressure (1 atm = 101.325 kPa), the partial pressure of oxygen is about 0.2095 atm ≈ 21.2 kPa ≈ 159 mmHg.

Can I enter mole fractions as percentages?

Yes. Toggle the "Use %" switch and enter values like 21 for 21%. The tool converts them to fractions (divides by 100) before applying Dalton's Law. The total must equal 100% in that mode.

Does this work for non-ideal gas mixtures?

Dalton's Law is exact for ideal gases. For real gases at high pressures or near condensation, the law is an approximation. For most practical purposes at moderate pressures the error is small.

How do I find total pressure from a known partial pressure?

Rearrange Dalton's Law: P_total = P_i / x_i. Enter the known partial pressure and the mole fraction of that gas component into the "Solve for Total Pressure" mode.

Partial Pressure Calculator

Name: Partial Pressure Calculator
Availability: InStock
Author: Nham Vu

Calculate partial pressure of any gas in a mixture using Dalton's Law — enter mole fractions and total pressure, or work backwards from partial pressure.

Calculation Mode

P_i = x_i × P_total

Dalton's Law of Partial Pressures

Total Pressure (P_total)

Gas Components Use %

Fraction

Enter your gas mixture values and click Calculate to see results.

Result Summary

Partial Pressures by Component

Gas	Mole Fraction	Partial Pressure	Share

Step-by-Step Solution

Summary

Calculate partial pressure of any gas in a mixture using Dalton's Law — enter mole fractions and total pressure, or work backwards from partial pressure.

How it works

Enter the total pressure of the gas mixture and choose the pressure unit (atm, kPa, Pa, mmHg, bar, or psi).
Add each gas component by typing its name and mole fraction (0 to 1). Mole fractions must sum to 1.
Click "Calculate" to compute the partial pressure for every gas using P_i = x_i × P_total.
The results table shows each gas's mole fraction, partial pressure, and percentage contribution.
Use the "Solve for Total Pressure" mode to find P_total when you know one partial pressure and its mole fraction.
Click Reset to clear all fields and start a new calculation.

Use cases

Determine the partial pressure of oxygen in air at a given atmospheric pressure.
Calculate partial pressures of gas components in industrial process streams.
Solve chemistry homework and exam problems involving Dalton's Law.
Find the partial pressure of water vapor in humid air from its mole fraction.
Verify gas mixture compositions in laboratory or engineering settings.
Determine the total pressure needed to achieve a target partial pressure.
Analyze respiratory gas mixtures for medical and diving applications.
Prepare for AP Chemistry, university general chemistry, or standardized science exams.

Frequently Asked Questions

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Last updated: 2026-05-28 · Reviewed by Nham Vu