Dalton's Law of partial pressures

Meaning: In an ideal gas mixture the particles do not interact, so each gas behaves as if it were alone in the container. Its partial pressure is the pressure it would exert alone at the same temperature and volume. Since PV = nRT, partial pressure depends on moles, so each gas contributes in proportion to its mole fraction.

Equation: Ptotal = P₁ + P₂ + P₃ + …, and Pᵢ = XᵢPtotal = nᵢRT/V. Pᵢ is the partial pressure of gas i (Pa), Xᵢ is its mole fraction (unitless), nᵢ is moles (mol), R = 8.314 J/(mol·K), T is temperature (K), V is volume (m³).

On the MCAT: Expect a gas collected over water, where Pgas = Ptotal − PH₂O (the vapor pressure of water at that temperature). Expect air at altitude: the O₂ fraction stays near 21%, but total pressure drops, so PO₂ drops. Gas exchange questions say O₂ and CO₂ diffuse from higher to lower partial pressure. Another setup adds an inert gas to a rigid container at constant temperature: the other partial pressures stay the same and the total rises.

Common confusion: Students think a heavier or larger gas contributes more pressure. At the same T and V, only moles matter, so equal moles of He and Xe have equal partial pressures. Mole fraction, not mass fraction, goes into Pᵢ = XᵢPtotal.