Which of the following salts of weak acids and weak bases on hydrolysis turns red litmus blue?
[K_a $for acetic acid = 1.8\times10^{-5}$ , $K_a$ for hydrofluoric acid = $6.8\times10^{-4}$ , $K_a$ for hydrogen cynide = $4.0\times10^{-10}$ , $K_a$ for formic acid $1.8\times10^{-4}$ and $K_b$ for ammonium hydroxide = $1.8\times10^{-5}$ ]

Question

Which of the following salts of weak acids and weak bases on hydrolysis turns red litmus blue?
[K_a

for acetic acid = 1.8\times10^{-5}

,

K_a

for hydrofluoric acid =

6.8\times10^{-4}

,

K_a

for hydrogen cynide =

4.0\times10^{-10}

,

K_a

for formic acid

1.8\times10^{-4}

and

K_b

for ammonium hydroxide =

1.8\times10^{-5}

]

Quantrex Academy · Accepted Answer

Correct answer: C. Ammonium cynide

A salt of a weak acid and a weak base forms a basic solution if the dissociation constant of the base (K_b

) is greater than the dissociation constant of the acid (K_a

). A basic solution turns red litmus blue.

For ammonium acetate:

K_a = 1.8 \times 10^{-5}

and

K_b = 1.8 \times 10^{-5}

. Since

K_a = K_b

, the solution is neutral.

For ammonium fluoride:

K_a = 6.8 \times 10^{-4}

and

K_b = 1.8 \times 10^{-5}

. Since

K_a > K_b

, the solution is acidic.

For ammonium cyanide:

K_a = 4.0 \times 10^{-10}

and

K_b = 1.8 \times 10^{-5}

. Since

K_b > K_a

, the solution is basic.

For ammonium formate:

K_a = 1.8 \times 10^{-4}

and

K_b = 1.8 \times 10^{-5}

. Since

K_a > K_b

, the solution is acidic.

Thus, ammonium cyanide forms a basic solution and turns red litmus blue.

Answer: Ammonium cynide

Which of the following salts of weak acids and weak bases on...