Question

Solubility product of a salt $AB$ is $1\times {10}^{-8}$ in a solution in which the concentration of $A^{+}$ ions is ${10}^{-3}M$. The salt will precipitate when the concentration of $B^{-}$ ions is kept:

• A. between ${10}^{-8}M$ to ${10}^{-7}M$
• B. between ${10}^{-7}M$ to ${10}^{-8}M$
• C. $>{10}^{-5}M$
• D. $<{10}^{-8}M$

Answer 1

The correct option is C $>{10}^{-5}M$

$AB\rightleftharpoons A^{+}+B^{-}$
$K_{sp}=\left[A^{+}\right]\left[B^{+}\right]$

Salt will precipitatecout if ionic concentration $>K_{sp}$

Therefore,

$\left[A^{+}\right]\left[B^{+}\right]>1\times {10}^{-8}$
$1\times {10}^{-3}\left[B^{-}\right]>1\times {10}^{-8}$

$\left[B^{-}\right]>\frac{1\times {10}^{-8}}{1\times {10}^{-3}}$ or $1\times {10}^{-5}$