Question

Solubility product of a salt AB, is 4 x 10 9 in a
solution in which concentration of A2+
is 10-3 M.
The salt will precipitate when the concentration of
B becomes
2 x 10 3 M
12) 2 x 10 M
13) 4 x 10-6 M
(4) 4 x 10-3 M​

Answer 1

Correction:

$\\\tt k_{sp} = 4* 10^{-9}$

Your value is way too large.

Solution:

Option D.

$\\\tt 4* 10^{-5}$

Theory Used:

The solubility product constant is given by,

$\\\tt K_{sp} = [A ^ {a+}][ B ^ {b-}]$

Therefore,

$\\\tt k_{sp} = [A^+][B^-]$

Using Question data:

$\\\tt k_{sp} = 10^{-3} [B^-] \\\tt 4 (10^{-9}) = 10^{-3} [B^-] \\\tt [B^-] = 4*10^{-6}$

The precipitation will start when $\\\tt [B^-] > 4 * 10^{-6}$

Which is in option (4)