Question

Which of the following are correct for $0.3M$ solution of calcium lactate in water if $K_a$ of lactic acid is $8.6\times {10}^{-4}$?

• A. The solution is basic
• B. The $pH$ of solution is $8.27$
• C. The $pH$ of solution is $8.42$
• D. $K_b$ of lactate ion (${Lac}^{-}$) is $1.16\times {10}^{-11}$

Answer 1

Answer: (A), (B), (D)

(A) The calcium lactate is a salt of weak base lactic acid and strong base calcium hydroxide.
These salts on hydrolysis form alkaline solution as strong base are completely dissociated and weak acid is partially dissociated.
Hence, option A is correct.
(B) and (C) $pH=7+0.5pKa+0.5logC$

$=7+0.5\times (-log8.6\times {10}^{-4})+0.5log0.3$

$=7+1.53-0.26$

$=8.27$
Thus, the option (B) is correct and the option (C) is incorrect.
(D) For lactate ion, $Kb=\frac{Kw}{Ka}=\frac{1\times {10}^{-14}}{8.6\times {10}^{-4}}=1.16\times {10}^{-11}$
Thus, the option D is correct.