Question

A weak monoacidic base dissociates to 1.5% in 0.001 M solution at 298 K . Calculate the dissociation constant of weak base.

• A. $2.25 \times 10^{-7}$
• B. $3.05 \times 10^{-7}$
• C. $2.5 \times 10^{-5}$
• D. $3.725 \times 10^{-6}$

Hint:

$K = C\alpha^2$ is valid when $\alpha$ is small (usually $< 5%$).

Answer 1

The Correct Option is A

Step 1: Given Data
Concentration ($C$) $= 0.001 \text{ M} = 10^{-3} \text{ M}$.
Degree of dissociation ($\alpha$) $= 1.5% = \frac{1.5}{100} = 0.015$.
Step 2: Formula
For a weak base, Ostwald's Dilution Law states:
$K_b = C \alpha^2$
Step 3: Calculation
$K_b = 10^{-3} \times (0.015)^2$
$K_b = 10^{-3} \times (2.25 \times 10^{-4})$
$K_b = 2.25 \times 10^{-7}$
Final Answer: (A)