Question

Calcium lactate $Ca(Lac{)}_2$ is salt of weak organic acid. A saturated solution of $Ca(Lac{)}_2$ contains 0.13 mole of this salt in 0.5 litre solution. The pOH of this solution is 5.60. Assuming a complete dissociation of salt, calculate $K_a$ lactic acid

• A.
• B.
• C.
• D.

Answer 1

The correct option is B

$Ca(Lac{)}_2\rightleftharpoons C{a}^2+2La{c}^{-1}$
$2\times 2\times 0.13$
$La{C}^{-1}+H_{2}O\rightleftharpoons HLac+O{H}^{-1}$
0.52-x x x

$K_h=\frac{x^2}{0.52-x}=\frac{x^2}{0.52}$ (assuming x is very small)
$\left[O{H}^{-}\right]={10}^{-5.6}=2.5\times {10}^{-6}=x$
$K_h=\frac{2.5\times {10}^{-6}\times 2.5\times {10}^{-6}}{0.52}=12.12\times {10}^{-12}$
$K_a=\frac{K_w}{K_h}=\frac{{10}^{-14}}{12.12\times {10}^{-12}}=8.26\times {10}^{-4}$