A 10.0-L stainless steel container was charged with 2.00 atm of hydrogen gas and 3.00 atm of oxygen gas. A spark ignited the mixture, producing water. What is the pressure in the tank at a constant temperature of 25°C?

Answer 1

Since these are gases, we can dispense with converting to moles; we may use pressure directly.
2 atm H2 will produce 2 atm H2O (as a gas which it isn't at 25C)
3 atm O2 will produce 6 atm H2O (as a gas but it isn't); therefore, H2 is the limiting reagent and O2 will be used but some will remain. How much will remain.
2 atm H2 reacts with 1 atm O2 to produce water (which as a liquid at 25 C we assume will not occupy much volume or change the pressure very much.)
We started with 3 atm O2, we used 1 atm, so 2 atm must remain. No pressure from H2 since all of it was used and no pressure from H2O since it is a liquid. I believe the answer is 2 atm pressure will exist in the container at 25 C. Check my thinking. You can double check the numbers by converting pressures to moles H2 and moles O2, redetermining the limiting reagent from moles, then convert back to pressure.