75L stainless steel container was charged with 3 atm of hydrogen gas and 4 atm of oxygen gas. a spark ignited the misture, producing water. what is the pressure of the tank at 25C? at 125C?

Answer 1

The 3 atm (partial pressure) of H2 will combine with 1.5 atm (partial pressure) of O2 to produce 3 atm (partial pressure) of H2O, with 2.5 atm (partial pressure) of O2 left over. Thus 7 atm of reactants produces 5.5 atm of products + unreacted O2.

I am assuming that the inital and final temperatures are what you have specified. This means that the heat of reaction will have been transfered out, as would be the case with a calorimeter. In that situation, with constant T and volume, the final pressure is proportional to the number of moles present, and that factor is 5.5/7 = 0.7857, at both temperatures.

Answer 2

I made a mistake in my previous answer by not considering the condensation of H2O. At 25C, almost all of the H20 produced will be in liquid form, so the actual pressure of the gas after cooling down the reaction products to 25 C will be 2.5/7 = 35.7% of the initial pressure. At 125 C, most (but not all) of the H2O produced by combustion will be in gaseous form. You will have to make use of data on the saturation pressure of H2O to get exact answers.