What are the partial pressures of the gases?
In a gaseous mixture, the partial pressure exerted by a component gas is the same as the pressure it would exert if it alone occupied the container.
The total pressure is the sum of the individual partial pressures:
And thus the partial pressure of any component gas is proportional to the mole fraction, the constant of proportionality
Clearly, this law assumes a mixture of non-reacting gases. And thus now we breathe dioxygen gas at a pressure of approx.