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Salt of Weak Acid and Strong Base

Calcium lacta...

Question

Calcium lactate is a salt of weak organic acid and is represented by $C a (L a c)_{2}$ . A saturated solution of $C a (L a c)_{2}$ contains 0.13 mol of this salt in 0.5 L solution. The pOH of this solution is 5.60. Assuming complete dissociation of the salt. Calculate $p K_{a}$ of lactic acid.

2.5

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5.7

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4.5

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3.1

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Solution

The correct option is D 3.1
We know, for salt of weak acid and strong base,
$l o g [H^{+}] = \frac{1}{2} l o g K_{w} + \frac{1}{2} l o g K_{a} - \frac{1}{2} l o g C$
$= p H = \frac{1}{2} (p K_{w} + p K_{a} + l o g C)$ ...(1)
where, $K_{w}$ = auto protolysis constant of water
$K_{a}$ = Dissociation constant of acid
C = Concentration of lactate ion = 0.26 mol/L
$C a (L a c)_{2} ⇌ C a^{2 +} + 2 L a c^{-}$
2 $\times$ 0.13 2 $\times$ 0.13 $\times$ 2
So, C = 0.52 mol/ L
Given, pOH = 5.6
$∴$ pH = 14 - 5.6 = 8.4
Put up in the given formula,
$8.4 = \frac{1}{2} (14 + p K_{a} + l o g 0.52$ )

$16.8 = 14 + p K_{a} + l o g 0.52$
Calculating for $p K_{a}$ ,
$p K_{a} = 2.8 - l o g 0.52 \approx 2.8 - l o g (0.5) p K_{a} = 2.8 + 0.3 = 3.1$

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Salt of Weak Acid and Strong Base

Standard XIII Chemistry

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Calcium lactate is a salt of weak organic acid and is represented by Ca(Lac)2. A saturated solution of Ca(Lac)2 contains 0.13 mol of this salt in 0.5 L solution. The pOH of this solution is 5.60. Assuming complete dissociation of the salt. Calculate pKa of lactic acid.

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