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Question
Ksp of Al(OH)3 is 1 × 10−15 M. At what pH does 1.0 × 10−3 M Al3+ precipitate on the addition of buffer of NH4Cl and NH4OH solution?
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Solution
\[\ce{Al(OH)3 ⇌ Al^{3+}_{( aq)} + 3OH^-_{( aq)}}\]
Ksp = [Al3+] [OH–]3
Al(OH)3 precipitates when
[Al3+] [OH–]3 > Ksp
(1 × 10−3) [OH–]3 > 1 × 10−15
[OH–]3 > 1 × 10−12
[OH–] > 1 × 10−4 M
[OH–] = 1 × 10−4 M
pOH = –log10 [OH–] = –log (1 × 10−4) = 4
pH = 14 – 4 = 10
Thus, Al(OH)3 precipitates at a pH of 10
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