Question

$K_{\text {sp }}$ of $Mg ( OH )_2$ is $4.0 \times 10^{-6}$. At what minimum $pH$, $Mg ^{2+}$ ions starts precipitating $0.01\, MgCl$

• A. $2+\log 2$
• B. $2-\log 2$
• C. $12+\log 2$
• D. $12-\log 2$

Answer 1

Answer: (C)

Precipitation starts when $Q_{ sp }=K_{ sp }$ :
$ K_{ sp }=\left[ Ca ^{2+}\right]\left[ \overset{\Theta}{O}H ^2\right]^2$
$\therefore[\overset{\Theta}{ OH }]=\left(\frac{4 \times 10^{-6}}{0.01}\right)^{1 / 2}=2 \times 10^{-2} $
$ pOH =-\log 2+2$
$ pH = i 4-[-\log 2+2]=12+\log 2$