Question

Calcium lactate is a salt of weak acid i.e., lactic acid having general formula Ca(LaC)2. Aqueous solution of salt has 0.3M concentration. pOH of solution is 5.60. If 90% of the salt is dissociated then what will be the value of pKa ?

• A. 2.8 – log (0.54)

Answer 1

Answer: (A)

2.8 – log (0.54)

Answer 2

Explanation of the solution:

1. The salt dissociates:
     Ca(LaC)₂ → Ca²⁺ + 2 LaC⁻
     Initial salt concentration = 0.3 M
     90% dissociation gives [LaC⁻] = 2 × 0.3 × 0.9 = 0.54 M.

2. For the weak conjugate base (lactate ion), using the hydrolysis relation:
     pOH = ½ [pKw − pKa − log (C)]
     Given pOH = 5.60 and pKw = 14, rearrange to:
     pKa = 14 − 2·(5.60) − log(0.54) = 2.8 − log(0.54).

Answer to the question:

pKa = 2.8 – log (0.54)  (option 0)