The electrolysis of a water solution of H2SO4 produces, after a certain period of time, 20L of H2, (T=25° and P=1); calculate the amount of Coulomb necessary for the hydrogen production and the moles of molecular oxygen developed.

Use PV = nRT and solve for n = number of mols H2 gas at 1 atm and 25 C. I get approximately 0.8 mol and that is 2*0.8 = 1.6 g H2 gas.

96,485 coulombs are required to produce 1 g H2. To produce 1.6 g will require 96,485 C x (1.6/1). How much oxygen will be produced.
2H2O ==> 2H2 + O2.
If you produce 0.8 mol H2, O2 must be 1/2 that.