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Introduction and Degree of Hydrolysis

If pK a 1, pK...

Question

If $p K_{a_{1}}, p K_{a_{2}} a n d p K_{a_{3}}$ of $H_{3} P O_{4}$ are 2.2, 7.2 and 12.3 respectively. The $p H$ of $0.1 M$ $N a_{2} H P O_{4}$ solution will be:

A

7.2

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B

12.3

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C

8.5

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D

9.75

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Solution

The correct option is D 9.75
$N a_{2} H P O_{4}$ is an amphiprotic salt. To calculate the pH of $N a_{2} H P O_{4}$
Considering the following reaction:
$H P O_{4}^{2 -} (a q) + H^{+} (a q) \to H_{2} P O_{4}^{-} (a q)$
$H P O_{4}^{2 -} (a q) - H^{+} (a q) \to P O_{4}^{3 -} (a q)$
$p H = \frac{1}{2} [p K_{a_{2}} + p K_{a_{3}}]$
Therefore, $p H = \frac{1}{2} [p K_{a_{2}} + p K_{a_{3}}]$ for $p H$ of $N a_{2} H P O_{4}$

$∴ p H = \frac{p K_{a_{2}} + p K_{a_{3}}}{2} = \frac{1}{2} (7.2 + 12.3) = 9.75$

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Similar questions

Q. A one litre solution contains

0.04 M

N a_{2} H P O_{4}

0.02 M

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l o g 2 = 0.3

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0.02 M

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is prepared. The electrolytic oxidation of

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milli-mole of the organic compound

R N H O H

is carried out in

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R N H O H + H_{2} O ⟶ R N O_{2} + 4 H_{4}^{+} e^{-}

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[Given: for

H_{3} P O_{4}, p K_{a_{1}} = 2.2; p K_{a_{2}} = 7.2; p K_{a_{3}} = 12

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K_{a 1}

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5, 8, 13

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Introduction and Degree of Hydrolysis

Standard XII Chemistry

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