Question

If the $pKa$ of lactic acid is $5$ , then the $pH$ of $0.005M$ calcium lactate solution at $25^{\circ }C$ is ______ $\times 10^{-1}$ (Nearest integer)
Lactic acid

Answer 1

Answer: 85

Concentration of calcium lactate $=0.005M$,: concentration of lactate ion $=(2\times 0.005)M$. Calcium lactate is a salt of weak acid $+$ strong base $\therefore$ Salt hydrolysis will take place.
$pH=7+\frac{1}{2}(pKa+\log C)$
$=7+\frac{1}{2}(5+\log (2\times 0.005))$
$=7+\frac{1}{2}[5-2\log 10]=7+\frac{1}{2}\times 3=8.5=85\times 10^{-1}$