Partial Pressure
Definition
The pressure one gas in a mixture would exert all by itself. The total pressure is simply the sum of all the partial pressures.
Technical definition
Partial pressure is the pressure a single component of a gas mixture would exert if it alone occupied the container's volume at the same temperature. Dalton's law states the total pressure equals the sum of partial pressures, with each partial pressure equal to its mole fraction times the total.
Example
In air at 1.00 atm, nitrogen's partial pressure is about 0.78 atm and oxygen's is about 0.21 atm, matching their mole fractions.
Common confusion
Partial pressure depends on mole fraction, not mass fraction. A heavy gas contributes less partial pressure per gram than a light gas does.