This concept has to do with partial pressure. Note the following: In a mixture of gases, each gas has a partial pressure which is the hypothetical pressure of that gas if it alone occupied the volume of the mixture at the same temperature. The total pressure of an ideal gas mixture is the sum of the partial pressures of each individual gas in the mixture. The formula is represented by the equation: Pt = P1 + P2 + ... Pt is the total pressure of all the combined gases P1 is the partial pressure for gas 1 P2 is the partial pressure for gas 2 It can go on until you include all the gases in the system.