Question

Which of the following solution when added to 1L of 0.1 M $C{H}_{3}COOH$ solution will cause no change either in the degree of dissociation of $C{H}_{3}COOH$or in the pH of the solution . $K_a=1.8\times {10}^{-5}$ for $C{H}_{3}COOH?$

• A. 3 mM HCOOH $(K_a=6\times {10}^{-4})$
• B. $0.1MC{H}_{3}COONa$
• C. $1.34mMHCI$
• D. $0.1MC{H}_{3}COOH$

Answer 1

Answer: (A), (C), (D)

The correct options are
A 3 mM HCOOH $(K_a=6\times {10}^{-4})$
C $1.34mMHCI$
D $0.1MC{H}_{3}COOH$

Given ,

$K_a=1.8\times {10}^5$

$C=0.1$

First calculate the hydrogen ion concentration in 0.1 M $C{H}_{3}COOH$ solution.

$\left[H^{+}\right]=\sqrt{K_a\times C}=\sqrt{1.8\times {10}^5\times 0.1}=1.34\times {10}^{-3}$

It is the concentration range of hydrogen ion above which pH will change.

The solutions in options A, C and D provide same hydrogen ion concentration.

Thus when these solutions are added, they will neither affect the degree of dissociation of $C{H}_{3}COOH$nor they will affect the pH of the solution.