The molecules of air frequently stumble into the walls of the vessel. These impacts apply energy on the walls of the vessel that isn't anything however termed as pressure. The pressure exerted by the gas is force per unit area. Although we know, the air is a mixture of gases such as nitrogen, oxygen, etc. and not a single compound. The pressure which is contributed by every gas is termed its partial pressure. Partial pressure can be explained as the pressure applied if that gas by itself had engaged the volume. Therefore, the entire pressure of air will be the summation of partial pressures of oxygen, nitrogen, etc.