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Q.

A volume of 50.00 mL of a weak acid of unknown concentration is titrated with 0.10 M solution of NaOH. The equivalence point is reached after 39.30 mL of NaOH solution has been added. At the half-equivalence point (19.65 mL), the pH is 4.85. Thus, initial concentrations of the acid and its $p K_{a}$ values are

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a

[HA] initial ${pK}_{a}$

0.079 M 2.93

b

[HA] initial ${pK}_{a}$

0.079 M 4.85

c

[HA] initial ${pK}_{a}$

0.1 M 4.85

d

[HA] initial ${pK}_{a}$

0.1 M 3.70

answer is D.

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Detailed Solution

$\underset{Acid}{M_{1} V_{1}} = \underset{Base}{M_{2} V_{2}}$

$M_{1} \times 50.0 = 0.10 \times 39.30 ∴ M_{1} = \frac{0.10 \times 39.30}{50} = 0.0786 M$

When half-equivalence point is reached

$[HA] = [A^{-}]$

$pH = {pK}_{a} + \log \frac{[A^{-}]}{[HA]}$

$∴ pH = {pK}_{a} = 4.85$

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A volume of 50.00 mL of a weak acid of unknown concentration is titrated with 0.10 M solution of NaOH. The equivalence point is reached after 39.30 mL of NaOH solution has been added. At the half-equivalence point (19.65 mL), the pH is 4.85. Thus, initial concentrations of the acid and its pKa values are