Question

If $Ksp$ of $Al ( OH )_{3}$ is $1.0 \times 10^{-15} \cdot M$. Find at what $pH$ does $1.0 \times 10^{-3} \cdot M Al ^{3+}$ precipitate on the addition of buffer of $NH _{4} Cl$ and $NH _{4} OH$ solution.

• A. 10
• B. 10.5
• C. 11
• D. 12

Answer 1

Answer: (A)

$Al ( OH )_{3}( s ) \rightleftharpoons Al ( aq )+3 O\overline{H} ( aq )$
$Ksp =\left[ Al ^{3+}\right][ O\overline H ]^{3}$
$Al ( OH )_{3}$ precipitates when
$\left[ Al ^{3+}\right][ O\overline{H} ]^{3}> Ksp$
$\left(1 \times 10^{-3}\right)\left[ OH ^{-}\right]^{3}>1.0 \times 10^{-15}$
$[ O \overline{ H }]^{3}>1 \times 10^{-12}$
$[ O \overline{ H }]>1 \times 10^{-4} M$
$pOH =-\log 1 \times 10^{-4}=4$
$pH =14-4=10$