partial pressure
Summary
Partial pressure is the pressure one gas in a mixture would exert if it alone filled the container at the same temperature. For ideal gases it equals the gas's mole fraction times the total pressure, and the partial pressures add up to the total (Dalton's law).
Detail
Meaning: Partial pressure is the pressure one gas in a mixture would exert if it alone filled the container at the same temperature. Ideal gas particles do not interact, so each gas contributes in proportion to its moles, and the contributions add to the total (Dalton's law). It also sets how much gas dissolves in a liquid (Henry's law) and which way a gas diffuses, from higher to lower partial pressure.
Equation: Pi = XiPtotal, where Pi is the partial pressure of gas i (Pa), Xi = ni/ntotal is its mole fraction (unitless), and Ptotal = P1 + P2 + … is the total pressure (Pa).
On the MCAT: You are given moles or percent composition and asked for one gas's partial pressure. Air is about 21% O₂, so PO₂ is about 0.21 atm at 1 atm total. For a gas collected over water, Pgas = Ptotal − PH₂O vapor. In altitude passages the O₂ fraction is about unchanged but Ptotal falls, so PO₂ falls.
Common confusion: A heavier or larger gas does not exert more pressure. At fixed T and V, only the number of moles matters, so equal moles of He and Xe have equal partial pressures.
Sources
- Chemistry: The Central Science
- Chemistry (Zumdahl)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.