Dalton's law of partial pressures states that the total pressure of a mixture of non-reacting gases is:

Dalton's law: in a mixture of non-reacting ideal gases, the total pressure $P_{\text{total}} = P_1 + P_2 + P_3 + \ldots$, where $P_i$ is the partial pressure (the pressure that gas $i$ would exert if it alone occupied the same volume at the same temperature). Equivalently, $P_i = x_i \cdot P_{\text{total}}$, where $x_i$ is the mole fraction of gas $i$. This works because in an ideal gas the molecules don't interact, so each species behaves independently.