The balanced chemical reaction is written as:
CaH2 + 2H2O ---> Ca(OH)2 + 2H2
We are given the hydrogen gas produced from the reaction. We use this amount as the starting point for the calculations.
4.5 g H2 ( 1 mol / 2.02 g) (1 mol CaH2 / 2 mol H2) ( 42.094 g / 1 mol ) = 46.87 g CaH2