The balanced chemical reaction will be:
H2 + Cl2 = 2HCl
We are given the amount of reactants and since chlorine is in excess we can say that all of the hydrogen supplied will be in complete reaction. This will be our starting point.
0.8 g H2 (1 mol H2 / 2.02 gH2) (2 mol HCl/1mol H2) ( 36.46 g HCl/1mol HCl) = 28.88 g HCl
Thus, the answer is 28.88g H2O.