The heat of solution of anhydrous $\mathrm{CuSO}_4$ and $\mathrm{CuSO}_4 \cdot 5 \mathrm{H}_2 \mathrm{O}$ are $-70 \mathrm{~kJ} \mathrm{~mol}^{-1}$ and $+12 \mathrm{~kJ} \mathrm{~mol}^{-1}$ respectively. The heat of hydration of $\mathrm{CuSO}_4$ to $\mathrm{CuSO}_4 \cdot 5 \mathrm{H}_2 \mathrm{O}$ is $-x \mathrm{~kJ}$. The value of $x$ is $\underline{\hspace{2cm}}$. (nearest integer).