Question

Calcium lactate is a salt of weak organic acid and strong base represented as Ca (LaC)₂ . A saturated solution of Ca (LaC)₂ contains 0.6 mole in 2 litre solution. pOH of solution is 5.60. If 90% dissociation of the salt takes place, then what is pK_a of lactic acid ?

• A. 2.8 – log (0.54)
• B. 2.8 + log (0.54)
• C. 2.8 + log (0.27)
• D. None of these

Answer 1

Answer: (A)

2.8 – log (0.54)

Answer 2

Ca (LaC)₂ ¾¾® Ca²⁺ + 2 LaC^–

Initial concentration 0.6/2 = 0.3 M

After dissociation [LaC] = 2 × 0.3 × 0.9 = 0.54 M

pH = $\frac{1}{2}$ [pK_w + pK_a + log (La C^–)]

14 – 5.6 = $\frac{1}{2}$ [14 + pK_a + log (0.54)]

pK_a = 2.8 – log (0.54)