Be sure to answer all parts. To study a key fuel-cell reaction, a chemical engineer has 20.0−L tanks of H2 and of O2 and wants to use up both tanks to form 31.0 mol of water at 23.8°C. (a) Use the ideal gas law to find the pressure needed in each tank. atm H2 atm O2 (b) Use the van der Waals equation to find the pressure needed in each tank. atm H2 atm O2