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 ?

1 Answer
Apr 10, 2018

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