The percentage composition of water in

i) ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}$    ii) ${\text{ZnSO}}_4\cdot 5{\text{H}}_2\text{O}$

(i) Molecular mass of  ${\text{Na}}_2{\text{CO}}_3\cdot 10{\text{H}}_2\text{O}=286.2\text{\hspace{0.17em}g}/\text{mol}$

Molar mass of water $=18.02\text{\hspace{0.17em}g/mol}$

${\text{Percentage\hspace{0.17em}of H}}_{\text{2}}\text{O}=\frac{10\times 18.02\text{\hspace{0.17em}g}}{286.2\text{\hspace{0.17em}g}}\times 100=62.9\%$

(ii) Molecular mass of ${\text{ZnSO}}_4\cdot 5{\text{H}}_2\text{O}=251.2\text{\hspace{0.17em}g/mol}$

${\text{Percentage\hspace{0.17em}of H}}_{\text{2}}{\text{O\hspace{0.17em}\hspace{0.17em}in ZnSO}}_{\text{4}}.5{\text{H}}_{\text{2}}\text{O,}=\frac{5\times 18.02}{251.2}\times 100=35.86\%$