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 efflorescent salt is one that loses `H_(2)O` to atmosphere. For the reaction
`CuSO_(4).5H_(2)O CuSO_(4).3H_(2)O(s)+H_(2)O(v)`
`K_(P)== 1.086xx10^(-4)`
`:. =1.042xx10^(-2) "atm" =7.92 mm`
Given at `25^(@)C, (i.e., 23.8)` which is greater than `7.92` mm and thus, reaction will proceed in the backward direction, i.e.,
`CuSO_(4).3H_(2)O+2H_(2)O(v) CuSO_(4).5H_(2)O(s)`
Thus, `CuSO_(4).5H_(2)O` will not act as efflorescent but on the contrary `CuSO_(4).3H_(2)O` will absorb moisture from the atmosphere under given conditions.