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A $100$ mL, $0.1=\mathrm{M}$ solution of ammonium acetate is diluted by adding $100 \mathrm{mL}$ of water. The pH of the resulting solution will be $\left(p K_{a}\right.$ of acetic acid is nearly equal to $\mathrm{pK}_{b}$ of $\mathrm{NH}_{4} \mathrm{OH}$ )
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The correct answer is:
7
Ammonium acetate is a salt of weak acid and weak base. When solutions of such salts diluted resulting pH of solution is calculated from following relationship
$$
\mathrm{pH}=7+\frac{1}{2} \mathrm{p} \mathrm{K}_{a}-\frac{1}{2} \mathrm{p} K_{\mathrm{b}}
$$
$\because p K_{a}=p K_{b}$
$\therefore \quad \mathrm{pH}$ of diluted solution $=7$
$$
\mathrm{pH}=7+\frac{1}{2} \mathrm{p} \mathrm{K}_{a}-\frac{1}{2} \mathrm{p} K_{\mathrm{b}}
$$
$\because p K_{a}=p K_{b}$
$\therefore \quad \mathrm{pH}$ of diluted solution $=7$
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