Question

A weak monobasic acid is \(0.08\%\) dissociated in \(0.02\)M solution. Calculate the dissociation constant of weak acid.

• A. \(1.28\times 10^{-8}\)
• B. \(1.50\times 10^{-8}\)
• C. \(1.1\times 10^{-8}\)
• D. \(1.60\times 10^{-8}\)

Hint:

Convert the percentage dissociation to a fraction and use the dissociation constant relation for a weak acid, the concentration times the square of the degree of dissociation.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept
For a weak monobasic acid, Ostwald's dilution law gives \(K_a=C\alpha^2\) when \(\alpha\) is small.

Step 2: Find the Degree of Dissociation
\(\alpha=\dfrac{0.08}{100}=8\times10^{-4}\).

Step 3: Calculation
\(K_a=0.02\times(8\times10^{-4})^2=0.02\times64\times10^{-8}\)
\(K_a=1.28\times10^{-8}\)

Step 4: Compare the options
This matches option (A).

Final Answer:
The dissociation constant is \(1.28\times10^{-8}\). \[ \boxed{\text{(A) }1.28\times 10^{-8}} \]