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A weak monobasic acid has an equivalent conductance of 8.0 mho cm^2 for an M/32 solution, while its limiting equivalent conductance at infinite dilution is 400 mho cm^2. What is the dissociation constant of the acid?

1. 1.25 × 10⁻⁶
2. 6.25 × 10⁻⁵
3. 1.25 × 10⁻⁴
4. 1.25 × 10⁻³

Correct answer: 1.25 × 10⁻⁶

Solution

The degree of dissociation is b1 = 8/400 = 0.02. For a weak monobasic acid, Ka = cb1^2/(1−b1), where c = 1/32 M. Substituting gives Ka 1.25 × 10⁻⁶.

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