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WBJEE 2022 Physics and Chemistry Paper

Question : 76 of 80

Marks: +1, -0

Which of the statements are incorrect?

$\mathrm{pH}$ of a solution of salt of strong acid and weak base is less than 7 .
$\mathrm{pH}$ of a solution of a weak acid and weak base is basic if $\mathrm{K}_b < \mathrm{K}_a$ .
$\mathrm{pH}$ of an aqueous solution of $10^{-8}(\mathrm{M})\,\mathrm{HCl}$ is 8 .
Conjugate acid of $\mathrm{NH}_2^{-}$ is $\mathrm{NH}_3$ .

Solution:

Hint:

\mathrm{pH}

of a solution of strong acid and weak base (say

\mathrm{NH}_4\mathrm{Cl}

) is less than 7 .

- For hydrolysis of salt of weak acid and weak base.

When

\mathrm{K}_a > \mathrm{K}_b, \;\; \mathrm{pH} < 7

\mathrm{pH}

10^{-8}\,\mathrm{M}\,\mathrm{HCl}(\mathrm{aq})

will be less than

7.

- Conjugate acid of

\mathrm{NH}_2^{-}

\mathrm{NH}_3

. This is a true statement.

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