Dalton's law of partial pressures states that the total pressure of a mixture of gases is the sum of all of the partial pressures of the individual gases.

The Dalton's law of partial pressures is expressed as : $P_{\mathrm{total}}=P_1+P_2+P_3\mathrm{...}{P}_n$
where:
P_total is the total pressure of a sample which contains a mixture of gases.
P₁, P₂, P₃, etc. are the partial pressures (in the same units) of the gases in the mixture.

Example 1:
A mixture of oxygen, hydrogen exerts a total pressure of 250 kPa. If the partial pressures of the oxygen is 162 kPa, what would be the partial pressure exerted by the hydrogen?

$P_{\mathrm{total}}=P_{\mathrm{O2}}+P_{\mathrm{H2}}$

$P_{\mathrm{H2}}=P_{\mathrm{total}}-P_{\mathrm{O2}}=250\mathrm{kPa}-162\mathrm{kPa}=88\mathrm{kPa}$