Q. The solubility product of iron (III) hydroxide is $1.6 \times 10^{- 39}$. If $X$ is the solubility of iron (III) hydroxide, which one of the following expression can be used to calculate $X$?
A) $K_{sp} = X^{4}$
B) $K_{sp} = 9X^{4}$
C) $K_{sp} = 27X^{3}$
D) $K_{sp} = 27X^{4}$
Explanation
(d)
The solubility product ($K_{sp}$) of a sparingly soluble salt at a given temperature may be defined as the product of the molar concentrations of its ions each raised to the power equal to its number of ions present in the equation representing the dissociation of one molecule of the salt.
$Fe(OH)_{3} \rightleftharpoons Fe^{3 +} + 3OH^{-}$
$X$ $X$ $3X$
$K_{sp} = \left\lbrack Fe^{3 +} \right\rbrack\left\lbrack OH^{-} \right\rbrack^{3}$
$\therefore$ $K_{sp} = X(3X)^{3} = 27X^{4}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.