What is Dalton's Law of Partial Pressures?
Dalton's law states that the total pressure of a gas mixture is the sum of the partial pressures of each individual gas — as if each gas occupied the entire volume alone. This principle is key to understanding mixtures of gases like air.
Dalton's law: P_total = P₁ + P₂ + P₃ + ... Each gas contributes pressure independently, unaffected by the others.
- •Gas A: 0.5 atm (alone in 10 L)
- •Gas B: 0.3 atm (alone in 10 L)
- •Gas C: 0.2 atm (alone in 10 L)
- •Total pressure = 0.5 + 0.3 + 0.2 = 1.0 atm
- •Each gas still exerts its partial pressure
- •Gases do not interact with each other
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Step-by-step worked examples
A mixture of gases contains 0.6 atm N₂, 0.2 atm O₂ and 0.1 atm Ar. What is the total pressure?
P_total = P_N2 + P_O2 + P_Ar P_total = 0.6 + 0.2 + 0.1 = 0.9 atm
Air at sea level is approximately 78% N₂, 21% O₂ and 1% Ar. If total pressure is 1 atm, what are the partial pressures?
P_N2 = 0.78 × 1 = 0.78 atm P_O2 = 0.21 × 1 = 0.21 atm P_Ar = 0.01 × 1 = 0.01 atm Check: 0.78 + 0.21 + 0.01 = 1.0 atm ✓
In a sealed container, H₂ has partial pressure 0.4 atm and O₂ has 0.5 atm. If you add He to reach 1.1 atm total, what is the partial pressure of He?
P_total = P_H2 + P_O2 + P_He 1.1 = 0.4 + 0.5 + P_He P_He = 1.1 − 0.9 = 0.2 atm
Flashcards
Quick quiz
Q1.In a 10 L container, N₂ at 0.6 atm and O₂ at 0.4 atm are mixed. Total pressure is…
Q2.Air is 78% N₂. If atmospheric pressure is 760 mmHg, partial pressure of N₂ is…
Q3.Dalton's law assumes…
Q4.Two gases in a container have partial pressures 3 atm and 2 atm. A third gas is added, bringing total to 6 atm. Partial pressure of the third…
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Common mistakes
Thinking gas volumes add up the same way pressures do. — Correct: Volumes DO add (V_total = V₁ + V₂), but partial pressures add independently of volume in Dalton's law.
Assuming gases interfere with each other's pressure. — Correct: Ideal gases don't interact — each gas's pressure is independent.
Confusing partial pressure with total pressure. — Correct: Partial = one gas's contribution; total = sum of all.
Using Dalton's law for reacting gases. — Correct: The law only applies to non-reacting, ideal gas mixtures.
FAQ
What is Dalton's law of partial pressures?
Total pressure = sum of partial pressures of each gas in the mixture. Each gas exerts pressure as if it alone filled the container.
Why does Dalton's law work?
Ideal gas molecules are far apart and don't interact, so each gas's pressure contribution is independent.
How do you calculate partial pressure from mole fraction?
Partial pressure = mole fraction × total pressure.
What is the partial pressure of oxygen in air?
Air is ~21% O₂, so at 1 atm total pressure, P_O2 ≈ 0.21 atm.