1 liter = 1 kg H20 = 1000 g H2O (because Density of Water = 1000 g/liter)

H = 1.01 g/mole (from periodic table)

O = 16.00 g/mole (from periodic table) (mol or mole is just a number, or quantity, if you wanted to count all the atoms that make a gram)

H2O = 18.02 g/mol

1000 g H2O / 18.02 g/mol = 55.5 moles of H20

H2O ---> (1/2)O2 + H2 [balanced chemical equation, excluding energy input]

55.5 ---> (1/2)55.5 + 55.5 [moles]

1000 g ---> 27.7 moles*32.00 g/mole + 55.5 moles*2.02 g/mole [grams]

Thus...

1000 g H2O ---> 886.4 g O2 + 112.1 g H2 (rounded)

if you did this calculation for gasoline, then you would find water has about 2.5x the amount of hydrogen (by mass or volume, i am not sure, because 1 liter of gasoline does not have the same mass as 1 liter of water, gasoline is lighter than water) ... meaning [1000 g *?or?* 1 liter] of gasoline has 44.84 grams of Hydrogen

so we have the mass of the gas from the separated water, if you want volume you have to know the density

Volume = Mass / Density

your density depends on pressure, I don't have the figures for that

A rough figure I know of is that it expands 1860 times when separated, so 1 liter of water = 1860 liters of gas, i think at 1 atm pressure

if your cell has a fixed volume, then density will increase with pressure

the exact calculations ca be done, you just have to get the right figures to start with and determine exactly what your situation is, for what you want to find out

provide more information on your question and i can look up some more exact values and try and do the exact calculations...