Why `Zn(II)` salt is not precipitated as `Zn(OH)_(2)` by ammonia solution in the presence of excess of ammonium

Explanation

`Zn^(2+)+2 NH_(3)+2H_(2)OhArrZn(OH)_(2)darr+2NH_(4)^(+)`
It is due to the lowering of `OH^(-)` ion concentration because of common ion effect of `NH_(4)^(+)` to such a value that the `K_(sp)` of `Zn(OH)_(2)` is not attained.