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Standard XII

Chemistry

Directive Influence of Substituents

Assertion :Gr...

Question

Assertion :Greater the value of $p K_{a}$ more is the acidic strength. Reason: Acidic strength increases with increase of dissociation constant of acid.

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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Assertion is incorrect but Reason is correct

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Solution

The correct option is D Assertion is incorrect but Reason is correct
The dissociation constant is given by $K_{a}$ .
We know that $p K_{a} = - log (K_{a})$
This implies that the greater the value of $p K_{a}$ , the smaller will be the value of $K_{a}$ .
Since $K_{a}$ is small, the acid dissociates weakly.
Greater the dissociation, greater will be the concentration of $H^{+}$ ions and hence, greater will be the strength of the acid. $K_{a}$ will be higher and $p K_{a}$ will be lower.
Therefore, the assertion is incorrect but the reason is incorrect.

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Assertion :Greater the value of pKa more is the acidic strength. Reason: Acidic strength increases with increase of dissociation constant of acid.

Assertion :Greater the value of $p K_{a}$ more is the acidic strength. Reason: Acidic strength increases with increase of dissociation constant of acid.