Total pressure of a gas mixture equals the sum of the partial pressures of its components.
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Dalton's law states that the total pressure of a mixture of non-reacting ideal gases equals the sum of the partial pressures of its components, P_total = P1 + P2 + ... . Each gas behaves as though it alone occupied the container at the same temperature, because the particles of an ideal gas do not interact. The partial pressure of a component equals its mole fraction multiplied by the total pressure, P_i = x_i P_total. This relation is used for gas mixtures such as air and for breathing gases in scuba diving. It is also used to correct the pressure of a gas collected over water.
Key ideas: Compute P_i = x_i P_total, where x_i is the mole fraction, or P_i = n_i RT/V. Mole fraction equals volume fraction for ideal gases, so a 21 percent oxygen mixture at 1 atm has 0.21 atm of oxygen. Add partial pressures only for gases in the same container at the same temperature.
Common mistakes and connections: Mistakes include using mass fractions instead of mole fractions and adding pressures of gases in separate containers. Partial pressure, not percentage, determines oxygen toxicity risk in diving, with trouble above about 1.4 atm of O2. Does not apply well to reacting gases. Connects to Henry's law, gas collection over water and Raoult's law.
Each gas in a mixture contributes its own partial pressure, P_total = sum of P_i; in air nitrogen supplies about 79 kPa of the 101.3 kPa total. The interactive lesson shows this as a live animation or chart.
Air at 1 atm, with oxygen at about 21 percent by volume, has an oxygen partial pressure of about 0.21 atm.
Total pressure of a gas mixture equals the sum of the partial pressures of its components.
Dalton's Law of Partial Pressures is a beginner topic in States of Matter & Solutions. It needs no earlier chemistry lessons and is a good starting point.
Study these earlier topics in States of Matter & Solutions first: Ideal Gas Law, Gas Constant, Absolute Zero.
The next topics in States of Matter & Solutions are Mole Fraction and Gas Collection Over Water.
Yes. The interactive lesson includes a self-check question on Dalton's Law of Partial Pressures, and the States of Matter & Solutions quiz draws on every topic in this area.
Dalton's Law of Partial Pressures is topic 16 of 109 in States of Matter & Solutions.