Under what pressure conditions `CuSO_(4).5H_(2)O` be efforescent at `25^(@)C`. How good a drying agent is `CuSO_(4).3H_(2)O` at the same temperature? Given
`CuSO_(4).5H_(2)O(s) hArr CuSO_(4).3H_(2)O(s)+2H_(2)O(v)`
`K_(p)=1.086xx10^(-4) atm^(2)` at `25^(@)C`. Vapour pressure of water at `25^(@)C` is `23.8` mm of Hg.

Explanation

An efforscent salt is one that loses `H_(2)O` to atmosphere for the reaction,
`CuSO_(4). 5H_(2)O(s) hArr CuSO_(4).3H_(2)O(s)+2H_(2)O(v)`
`K_(p)=(P'_(H_(2)O))^(2)=1.086xx10^(-4)`
`:. P'_(H_(2)O)=1.042xx10^(-2) "atm" =7.92 mm`
`:' "Given" P'_(H_(2)O)` at `25^(@), (i.e., 23.8 mm Hg) lt 7.92 mm`
and thus, reaction will proceed in forward direction, i.e., `CuSO_(4).5H_(2)O(s) hArr CuSO_(4).3H_(2)O(s)+2H_(2)O(v)`
Thus, `CuSO_(4).5H_(2)O` will acts as efflorescent but on the contrary `CuSO_(4).3H_(2)O` will absorb moisture from the atmosphere under given conditions. The salt `CuSO_(4).5H_(2)O` will effloresce only on a day when the aqueous tension or partial pressure of moisture in the air is lesser than `7.92 mm` of relative humidity of air at `25^(@)C=7.92/23.8=0.333` or `33.3%`