In the early 1800's John Dalton described the total pressure of a mixture of gases as the sum of the partial pressures of all gases in the mixture.
If all of the oxygen in a sample of air were removed by a chemical process, the total pressure in the room would fall. The amount of decrease would be equal to oxygen's portion or its partial pressure. Knowing the average composition of Earth's standard atmosphere, we would expect the final pressure in this case to be 79% its original value. The total minus the roughly 21% oxygen concentration.
