Question

Calcium lactate is a salt of weal organic acid and strong base represented as $Ca(LaC{)}_2$. A saturated solution of $Ca(LaC{)}_2$ contains $0.6$ mole in $2$ litre solution. $pOH$ of solution is $5.60$. If $90\text{\%}$ dissociation of the salt takes place then what is $p{K}_a$ of lactic acid?

• A. $2.8-\log \left(0.54\right)$
• B. $2.8+\log \left(0.54\right)$
• C. $2.8+\log \left(0.27\right)$
• D. None of these

Answer 1

The correct option is A $2.8-\log \left(0.54\right)$

The Molar concentration of the salt is $\frac{0.6}{2}=0.3M$

The hydrogen ion concentration is $2\times 0.3\times 0.9=0.54M$

The expression for pH is $pH=7+0.5pKa+0.5logC$

$pH=14-pOH=14-5.6=8.4$

Substitute values in the above expression, we get
$8.4=7+0.5pKa+0.5log0.54$
$p{K}_a=2.8-log\left(0.54\right)$