Determine the mass of hydrogen gas if 36g of water undergoes electrolysis to produce hydrogen and oxygen.

Write and balance the equation.

Stoichiometry problem are worked this way.
2H2 + O2 ==> 2H2O

Convert 36 g H2O to moles. moles = grams/molar mass

Using the coefficients in the balanced equation, convert moles H2O to moles H2 gas.

Now convert moles H2 gas to grams. g = moles x molar mass.