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Arrhenius Acid

The pH of a...

Question

The $p H$ of a buffer solution made by mixing $25$ mL of $0.02 M$ ${N H}_{4} O H$ $({p K}_{b} = 4.8)$ and $25$ mL of $0.2 M$ ${N H}_{4} C l$ at $25^{\circ} C$ is:

5.8

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8.2

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4.8

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3.8

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Solution

The correct option is B $8.2$
It is an example of basic buffer solution containing weak base ammonium hydroxide and its salt ammonium chloride with strong acid (HCl).

$p O H = p K_{b} + l o g \frac{[N H_{4} C l]}{[N H_{3} O H]}$

$p O H = 4.8 + l o g \frac{25 \times 0.2}{25 \times 0.02}$

$p O H = 4.8 + l o g 10 = 4.8 + 1 = 5.8$

$∴ p H = 14 - p O H = 14 - 5.8 = 8.2$

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