For the equilibrium
${\mathrm{CuSO}}_4\cdot 5{\mathrm{H}}_2\mathrm{O}\left(s\right)\rightleftharpoons {\mathrm{CuSO}}_4\cdot 3{\mathrm{H}}_2\mathrm{O}\left(s\right)+2{\mathrm{H}}_2\mathrm{O}\left(g\right)$ ${\mathrm{K}}_{\mathrm{p}}=2.25\times {10}^{-4}{\mathrm{atm}}^2$ and vapour pressure of water is 22.8 Torr at 298 K.${\mathrm{CuSO}}_4\cdot 5{\mathrm{H}}_2\mathrm{O}\left(s\right)$ is efflorescent (i.e., loses water) 

when relative humidity is :

Here, the pressure on the water molecule,

$\begin{array}{l}{P}_{{\mathrm{H}}_2\mathrm{O}}^2=2.25\times {10}^{-4}\\ \Rightarrow P_{{\mathrm{H}}_2\mathrm{O}}=1.5\times {10}^{-2}\mathrm{atm}\end{array}$

Relative humanity$=\frac{1.5\times {10}^{-2}}{22.8}\times 760=0.5\text{ or }50\mathrm{\%}$

If Relative humanity $>50\mathrm{\%} {\mathrm{Rx}}^{\mathrm{n}}$ goes backward.

If Relative humanity $<50\mathrm{\%} {\mathrm{CuSO}}_4\cdot 5{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)$ becomes efflorescent.