If the pKa of lactic acid is 5, then the pH of 0.005 M calcium lactate solution at 25°C is ______ × 10 - 1 (Nearest integer). [2023]

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Solution

Q.
If the pKa of lactic acid is 5, then the pH of 0.005 M calcium lactate solution at 25°C is ______ $\times 10^{- 1}$ (Nearest integer). [2023]

Ans.
(85)

$\begin{matrix} {Ca(Lac)}_{2} & ⟶ & {Ca}^{2 +} & + & 2 {Lac}^{-} \\ 5 \times 10^{- 3} & 5 \times 10^{- 3} & 2 \times 5 \times 10^{- 3} \\ = 10^{- 2} M \end{matrix}$

Calcium lactate is a weak acid and strong base salt, so

$pH = 7 + \frac{1}{2} p K_{a} + \frac{1}{2} \log C$

   $= 7 + \frac{1}{2} \times 5 + \frac{1}{2} \log 10^{- 2}$

   $= 7 + 2.5 - 1$

   $= 6 + 2.5$

   $= 8.5$

   $= 85 \times 10^{- 1}$

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