Question

Solubility product of Al(OH)3 and Zn(OH)2 are 8.5*10^-23 and 1.8*10^-14 If NH4OH is added to a solution containing Al3+ and Zn2+ ions, then substance precipitated first is ?

Answer 1

the substance precipitated first is `Al(OH)_3`

Explanation:

`Al(OH)_3 = Al^(3+)+ 3 OH^-`
`Kps=[Al^(3+)] xx [OH^-]^3= 8.5 xx 10^(-23)`
if you suppose `[Al^(3+)]= 10^(-2)`
`[OH^-]=root(3)((kps)/([Al^(3+)]))=root(3)(8.5 xx 10^(-23)/(10^(-2)))=2 xx 10^-7`
pOH= 6,7
pH= 7,3
`Zn(OH)_2 = Zn^(2+)+ 2 OH^-`
`Kps=[Zn^(2+)] xx [OH^-]^2= 1.8 xx 10^(-14)`
if you suppose `[Zn^(2+)]= 10^(-2)`
`[OH^-]=root(2)((kps)/([Zn^(2+)]))=root(2)(1.8 xx 10^(-14)/(10^(-2)))=1,3 xx 10^-6`
pOH =5,9
pH = 8,1
the substance precipitated first is `Al(OH)_3`

in truth your data is wrong: Kps `Al(OH)_3` is `2 xx 10^(-31)` and it begins to precipitate at pH about 4