Question

When NH₄ Cl and NH₄OH are added to a solution containing both, Fe³⁺ and Ca²⁺ ions, which ion is precipitated first and why?

Answer 1

`NH_4OH ⇌ NH_4^+ + OH^(-)`

`NH_4Cl -> NH_4^+ + Cl^(-)`

`Fe^(3+) + 3OH^(-) -> Fe(OH)_3`

`Ca^(2+) + 2OH^(-) -> Ca(OH)_2`

∵ `K_"sp" [Fe(OH)_3)]` is very less compared to `K_"sp"[Ca(OH)_2]` hence `Fe(OH)_3` will be precipiated and `Ca(OH)_2` remain is solution.