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The $pK_{a}$ of a weak acid $\left ( HA \right )$ is 4,5.The $pOH$ of an aqueous bufferd solution of $HA$ in which 50% of the acid is ionized is

Option 1)
$7.0$

Option 2)
$4.5$

Option 3)
$2.5$

Option 4)
$9.5$

Answers (1)

best_answer

As we learnt in

Buffer Solutions -

The solutions which resist change in pH on dilution or with addition of small amounts of acid or alkali are called buffer solution.

- wherein

Many body fluids e.g. blood

and

p(Kw) -

$K_{w}=[H_{2}O^{+}]\:[H\bar{O}]=10^{-14}$

Although $K_{w}$ may change with temperature the variation in pH with temperature are so small that we often ignore it.

- wherein

$p(K_{w})=-logK_{w}$

$=-log{[H_{3}O^{+}]\:[\bar{O}H]}$

$=-log10^{-14}$

$p(K_{w})=p(H)+p(OH)$

$=14$

For acidic buffer, $pH=pK_{a}+log\, \frac{\left [ A^{-} \right ]}{\left [ HA \right ]}$

When the acid is 50% dissociated , $\left [ A^{-} \right ]=\left [ HA \right ]$

$or\; \; \; pH=pK_{a}+log1\; \; \; or\; \; pH=pK_{a}$

$Given\; \; pK_{a}=4.5\; \; \therefore pH=4.5$

$\therefore \; \; pOH=14-4.5=9.5$

Option 1)

$7.0$

This option is incorrect

Option 2)

$4.5$

This option is incorrect

Option 3)

$2.5$

This option is incorrect

Option 4)

$9.5$

This option is correct

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prateek

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