The weight of hydrogen gas obtained from 42 g of ${\mathrm{CaH}}_2$ by treatment with water is:

Water reacts with calcium hydride in the following manner:

${\mathrm{CaH}}_2+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{Ca}{\left(\mathrm{OH}\right)}_2+2{\mathrm{H}}_2$

One mole of calcium hydride gives two moles of hydrogen gas. The number of moles in 42 g calcium hydroxide is;

$\mathrm{no}. \mathrm{of} \mathrm{moles}=\frac{\mathrm{given} \mathrm{mass}}{\mathrm{molar} \mathrm{mass}}=\frac{42}{42}=1$

Thus two moles of hydrogen is formed. The mass of two mol hydrogen gas is;

$\mathrm{no}. \mathrm{of} \mathrm{moles}\times \mathrm{molar} \mathrm{mass}=2\times 2=4 \mathrm{g}$