Question

Which of the following will not dissolve in a hot mixture of $NaOH$ and $H_2{O}_2$?

• A. $Fe(OH{)}_3$
• B. $Al(OH{)}_3$
• C. $Cr(OH{)}_3$
• D. $Zn(OH{)}_2$

Answer 1

The correct option is A $Fe(OH{)}_3$

Most hydroxide salts are insoluble. For example, the solubility of ${Fe}^{+3}$ in basic solution is governed by the reaction shown in (eq 1), from which the corresponding equilibrium constant expression can be derived (eq 2). The equilibrium constant for the reaction $K_{sp}$ is $6.3X{10}^{-38}$ which says that $Fe{\left(OH\right)}_3$ is sparingly soluble under most conditions. Equation 3 says that the solubility of ferric ionrapidly decreases as the solution becomes more basic.

$Fe{\left(OH\right)}_3\rightleftharpoons {Fe}^{+3}+3{OH}^{-}------\left(1\right)$

$K_{sp}=\left[{Fe}^{+3}\right]{\left[{OH}^{-}\right]}^3------\left(2\right)$

$Solubility=\left[{Fe}^{+3}\right]=\frac{K_{sp}}{{\left[{OH}^{-}\right]}^3}------\left(3\right)$

Hence, answer is $A$