Physical Chemistry Problem!! The total pressure of a mixture of oxygen and hydrogen is 1 atm. The mixture is ignited and the water is removed. The remaining gas is pure hydrogen and exerts a pressure of 0.4 atm when measured at the same values of T and V as the original mixture. What was the composition of the original mixture in mole percent? I started by using the equation P=nRT/V. Since the V, R and T are the same before and after the reaction I solved the equation for P/n = RT/V so the two equations (for before and after the reaction) could be set equal to each other. Then I got stuck! Please Help!