If `CuSO_(4)`.`5H_(2)O_((s))hArrCuSO_(4)`.`3H_(2)O_((s)) + 2H_(2)O_((l)) K_(p) = 1.086xx10^(-4) atm^(2)` at `25^(@)C`. The efflorescent nature of `CuSO_(4)`.`5H_(2)O` can be noticed when vapour pressure of `H_(2)O` in atmosphere is