Assertion :In a titration of weak acid and $N a O H$ , the pH at half equivalence point is $p K_{a}$ Reason: At half equivalence point, it forms an acidic buffer and the buffer capacity is maximum where [acid]=[salt]

Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

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Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

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Assertion is correct but Reason is incorrect

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Both Assertion and Reason are incorrect

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Solution

The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
$p H = p K_{a} + l o g \frac{[A^{-}]}{[H A]}$

At the half-equivalence point, say we have 10 Mols of weak acid, and so there will be 5 Mols of strong base as the name suggests("half equivalence"). So we know that the 10 Mols will be neutralized to 5 Mols of WA by the SB. If we do this then the CB is also now equal to the WA since it will increase by 5 Mols. WA = CB.

$p H = p K_{a} + l o g [1]$
$= p K_{a}$

So this is why the $p H = p K_{a}$ at the half-equivalence point.

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Assertion :In a titration of weak acid and NaOH, the pH at half equivalence point is pKa Reason: At half equivalence point, it forms an acidic buffer and the buffer capacity is maximum where [acid]=[salt]

Assertion :In a titration of weak acid and $N a O H$ , the pH at half equivalence point is $p K_{a}$ Reason: At half equivalence point, it forms an acidic buffer and the buffer capacity is maximum where [acid]=[salt]