The pK_b value of ammonium hydroxide is 4.75. An aqueous solution of ammonium hydroxide is titrated with HCl. The pH of the solution at the point where half of the ammonimum hydroxide has been neutralized will be :

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7.50

9.25

4.75

8.25

answer is A.

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Detailed Solution

In the solution of NH₄OH, following equilibrium exists.

$N{H_4}OH \rightleftharpoons N{H_4}^ + (aq) + O{H^ - }(aq)$

As the solution of NH₄OH is treated with HCl, neutralisatin occurs and the equilibrium moves forward.

At the point, where half of the ammonium hydroxide has been neutalised, $\left[ {N{H_4}^ + } \right] = \left[ {N{H_4}OH} \right]$ and the system behaves as a basic buffer pOH of, such a system is given by,

$pOH = p{K_b} + \log \frac{{\left[ s \right]}}{{\left[ b \right]}}$

~~$\begin{gathered} \Rightarrow pOH = 4.75 \\ \therefore pH = 14 - 4.75 = 9.25 \\ \end{gathered}$~~

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