Question

The solubility product of a salt $AB$ is $1\times {10}^{-8}$. In a solution, in which concentration of $A$ is ${10}^{-3}M$, $AB$ will precipitate when the concentration of $B$ will be:

• A. ${10}^{-7}M$
• B. ${10}^{-4}M$
• C. ${10}^{-5}M$
• D. ${10}^{-6}M$

Answer 1

Answer: (B)

${10}^{-4}M$

For a saturated solution $K_{sp}=\left[A^{+}\right]\left[B^{-}\right]$

${10}^{-8}=\left[{10}^{-3}\right]\left[B^{-}\right]$

$\left[B^{-}\right]={10}^{-5}$

The precipitation will occur when $Q_{sp}$ is greater than $K_{sp}$ and it will possible when $\left[B^{-}\right]<{10}^{-5}$.

Only option (b) is correct.