Question 35

Given below are two statements : Statement (I) : Aqueous solution of ammonium carbonate is basic.

Statement (II) : Acidic/basic nature of salt solution of a salt of weak acid and weak base depends on $$K_a$$ and $$K_b$$ value of acid and the base forming it.

In the light of the above statements, choose the most appropriate answer from the options given below :

Solution

Statement I is correct: Ammonium carbonate, $$(\text{NH}_4)_2\text{CO}_3$$, is a salt of a weak base ($$\text{NH}_4\text{OH}$$) and a weak acid ($$\text{H}_2\text{CO}_3$$). Because the base dissociation constant ($$K_b \approx 1.8 \times 10^{-5}$$) is greater than the acid dissociation constant ($$K_{a2} \approx 4.7 \times 10^{-11}$$), the solution is basic ($$\text{pH} > 7$$).

Statement II is correct: The $$\text{pH}$$ formula for a salt of a weak acid and a weak base is:

$$\text{pH} = 7 + \frac{1}{2}(\text{p}K_a - \text{p}K_b)$$

This formula proves that whether the solution is acidic, basic, or neutral depends entirely on the relative values of $$K_a$$ and $$K_b$$.

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