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 `35^(@)C`. Vapoure pressure of water at `25^(@)C` is `23.8 mm` of Hg.

Explanation

An efforescent 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))=1.268xx10^(-3)`
`:. P'_(H_(2)O)=0.0356 "atm"=27.0 mm Hg`
`:' "Given" P'_(H_(2)O)` at `35^(@)C`, (i.e. 25.0 mm Hg) lt `27.0 mm Hg` 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