If the pKa of lactic acid is 5, then the pH of 0.005 M calcium lactate solution at $25^{\circ}\mathrm{C}$ is ____ $\times 10^{-1}$ (Nearest integer) Lactic acid

JEE Main · 2023 · 24 Jan 2023 · Shift 2 · Equilibrium

If the pKa of lactic acid is 5, then the pH of 0.005 M calcium lactate solution at

2 5^{\circ} C

is ____

\times 1 0^{- 1}

(Nearest integer)

Lactic acid

Answer: 85

Solution

Calcium lactate is the salt of weak lactic acid and the strong base calcium hydroxide, so its lactate ion hydrolyses to give a basic solution. Each formula unit releases two lactate ions, so the lactate concentration is

2 \times 0.005 = 0.01 M

. For the salt of a weak acid and strong base the balance gives

pH = 7 + \frac{1}{2} (p K_{a} + lo g C) = 7 + \frac{1}{2} (5 + lo g 0.01) = 7 + \frac{1}{2} (3) = 8.5

. Expressed as a multiple of

1 0^{- 1}

, the pH is

85 \times 1 0^{- 1}

JEE Main · 2023 · 24 Jan 2023 · Shift 2 · Integer answer

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