Question

The solubility product of iron (III) hydroxide is $1.6\times {10}^{-39}$. If $X$ is the solubility of iron(III) hydroxide, then which one of the following expressions can be used to calculate $X$?

• A. $K_{sp}=X^4$
• B. $K_{sp}=9X^4$
• C. $K_{sp}=27X^3$
• D. $K_{sp}=27X^4$

Answer 1

The correct option is D $K_{sp}=27X^4$

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

$Ksp=\left[F{e}^{3+}\right][O{H}^{-}{]}^3$

$K_{sp}=\left(X\right){\left(3X\right)}^3=27X^4$

Hence, option D is correct.