Dalton's Law of Partial Pressures
The total pressure is the sum of each gas's partial pressure.
About this topic
Dalton's law states that the total pressure of a mixture of gases is the sum of the partial pressures each gas would exert alone: P_total = P1 + P2 + P3 + .... The partial pressure of a gas equals its mole fraction times the total pressure.
In an ideal gas mixture, particles do not interact, so each gas behaves as if it were alone in the container. Air at 1 atm is about 78 percent nitrogen, 21 percent oxygen and 1 percent argon and other gases, so the partial pressure of oxygen is about 0.21 atm. Mole fraction is moles of one gas divided by total moles. When a gas is collected over water, the total pressure includes water vapour, whose partial pressure at that temperature must be subtracted to get the pressure of the dry gas.
Worked example: oxygen collected over water at 25 degrees Celsius with total pressure of 101.3 kPa. Water vapour pressure is 3.17 kPa, so the oxygen pressure is 98.1 kPa. Divers use mixtures like heliox and trimix, because at depth high partial pressures of oxygen or nitrogen become toxic or narcotic. Mountaineers struggle because the partial pressure of oxygen falls with altitude.
Misconceptions are that heavier gases exert more pressure or that gases in a mixture compete. Pressure depends on the number of particles, not their identity.
Key ideas
- P_total = P₁ + P₂ + …
- Mole fraction × total pressure
- Used in scuba mixes
Quick check
Use cases: where Dalton's Law of Partial Pressures helps
- Homework: review the key idea behind Dalton's Law of Partial Pressures before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Dalton's Law of Partial Pressures without notes.
- Visual learning: play with the interactive lab to see how Dalton's Law of Partial Pressures changes when you adjust its inputs.
- Teaching a friend: explain Dalton's Law of Partial Pressures in your own words, then compare your version with the key ideas.
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Frequently asked questions
What is Dalton's Law of Partial Pressures?
The total pressure is the sum of each gas's partial pressure.
What will I learn in Dalton's Law of Partial Pressures?
P_total = P₁ + P₂ + …. Mole fraction × total pressure. Used in scuba mixes.
Is Dalton's Law of Partial Pressures part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Dalton's Law of Partial Pressures lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Dalton's Law of Partial Pressures?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
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