Molar mass
CaH₂ = 42.0 g/mol
H₂ = 2.0 g/mol
Balanced chemical equation :
CaH2 + 2 H₂O = Ca(OH)₂ + 2 H₂
42.0 g CaH₂ --------------------- 4.0 g H2
? g CaH₂ ---------------------- 4.550 g H2
mass = 4.550 * 42.0 / 4
mass = 191,1 / 4
mass = 47.775 g of CaH₂
hope this helps!