Question

What is the solubility of $Al ( OH )_{3},\left( K _{ sp }=\right.$ $\left.1 \times 10^{-33}\right)$ in a buffer solution $pH =4$ ?

• A. $10^{-3} M$
• B. $10^{-6} M$
• C. $10^{-4} M$
• D. $10^{-10} M$

Answer 1

Answer: (A)

$pH =4$
$\Rightarrow \left[ H ^{+}\right]=10^{-4} M$
$\Rightarrow \left[ OH ^{-}\right]=10^{-10} M$
$Al ( OH )_{3} \rightleftharpoons Al ^{+3}+3 OH ^{-}$
$K _{ sp }\left( Al ( OH )_{3}\right)=\left[ Al ^{+3}\right]\left[ OH ^{-}\right]^{3}$
$\left[ Al ^{3+}\right]\left[ OH ^{-}\right]^{3}=1 \times 10^{-33}$
$\left[ Al ^{3+}\right]\left(10^{-10}\right)^{3}=1 \times 10^{-33}$
$\Rightarrow \left[ Al ^{+3}\right]=10^{-3} M$