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Acidic Buffer Action

8g of NaOH wa...

Question

8g of $N a O H$ was dissolved in 1 L of a solution containing one mole of acetic acid $C H_{3} C O O H$ and one mole of sodium acetate $C H_{3} C O O N a$ . Find the pH of the resulting solution.
The dissociation constant for $C H_{3} C O O H$ is $1.8 \times 10^{- 5}$ .

6.88

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4.92

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3.68

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5.76

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Solution

The correct option is B 4.92
Concentration of $N a O H = \frac{8}{40} = 0.2 m o l L^{- 1}$
$C H_{3} C O O H (a q) + N a O H (a q) \to C H_{3} C O O N a (a q) + H_{2} O (l) I n i t i a l : 1 0.2 10 E q u i l i b r i u m : (1 - 0.2) 01 + 0.2 0.2$
$p K_{a} = - l o g (K_{a}) p K_{a} = - l o g (1.8 \times 10^{- 5}) p K_{a} = (5 - 0.26) = 4.74$

$[C H_{3} C O O H]_{e q b} = 0.8 M [C H_{3} C O O N a]_{e q b .} = 1.2 M p H = p K_{a} + l o g \frac{[s a l t]}{[a c i d]} p H = p K_{a} + l o g (\frac{[C H_{3} C O O N a]}{[C H_{3} C O O H]}) p H = 4.74 + l o g (\frac{1.2}{0.8}) p H = 4.74 + l o g (1.5) = (4.74 + 0.18) p H = 4.92$

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