25°C তাপমাত্রায় Fe(OH)₃ এর দ্রাব্যতা 2.1153 × 10^-8 g dm^-3 হলে ঐ তাপমাত্রায় দ্রাব্যতা গুণফল কত? 

[Fe=55.85]

Here,

$\begin{aligned} {\left[\mathrm{Fe}^{3+}\right] } & =\left[\mathrm{OH}^{-}\right]=S \\ & =2.1153 \times 10^{-8} \mathrm{gdm}^{-3} \\ & =\frac{2.1153 \times 10^{-8}}{106.85} \mathrm{molL}^{-1} \\ & =1.98 \times 10^{-10} \mathrm{molL}^{-1} \\ {\left[1 \mathrm{dm}^{-3}\right.} & \left.=1 \mathrm{~L}^{-1}\right] \end{aligned}$

$\mathrm{Fe}(\mathrm{OH})_{3} \rightleftharpoons \mathrm{Fe}^{3+}(\mathrm{aq})+3 \mathrm{OH}^-(\mathrm{aq})$

$\begin{aligned} \therefore K_{s p} & =\left[\mathrm{Fe}^{3+}\right] \times\left[3 \mathrm{OH}^{-}\right]^{3} \\ & =s \times(3 \mathrm{~s})^{3} \\ & =27 \times s^{4} \\ & =27 \times\left(1.98 \times 10^{-10}\right)^{4} \\ & =4.147 \times 10^{-38}\end{aligned}$