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  4. The composition of an acidic buffer mixture made up of HA and NaA of total molarity 0.29 having pH =4.4 and Ka=1.8 × 10-5 in terms of concentration of salt and acid respectively is

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Q. The composition of an acidic buffer mixture made up of $HA$ and $NaA$ of total molarity $0.29$ having $pH =4.4$ and $K_{a}=1.8 \times 10^{-5}$ in terms of concentration of salt and acid respectively is

Equilibrium

Solution:

$pH =-\log \,K_{a}+\log \frac{[\text { Salt }]}{[\text { Acid }]}$

Let $a\, mol \,L^{-1}$ be concentration of salt, then concentration of acid $=(0.29-a)$

$4.4=-\log \left(1.8 \times 10^{-5}\right)+\log \frac{a}{(0.29-a)} $

$\Rightarrow a=0.09$

$\therefore [ Salt ]=0.09 M$ ; [ Acid $]=0.29-0.09=0.20\, M$