# What is the partial pressure of oxygen?

At sea level, since dioxygen 21% of the the atmosphere, the partial pressure of dioxygen is $0.21 \times 1 \cdot a t m = 0.21 \cdot a t m$. What is the partial pressure of dinitrogen if dinitrogen constitutes 79% of the atmosphere?
Note that, as here and as always, the total pressure is the sum of the individual partial pressures: here $1 \cdot a t m = \left(0.21 + 0.79\right) \cdot a t m$ as required.