Question

A weak monobasic acid is 2% dissociated in its 0.01 M solution. What is the dissociation constant of a weak acid?

Options

• 3 × 10^-6

• 2 × 10^-6

• 25 × 10^-6

• 4 × 10^-6

Answer 1

4 × 10^-6 

Explanation:

Let us take a reaction

	\[\ce{HA -> H+ + A-}\]
At t = 0	C	0	0
At t = t	C - Cα	Cα	Cα

Given, C = 0.01 M

α = 2% = 0.02

Dissociation constant `("K"_"a") = ("C"alpha)^2/(1 - alpha)`    ...[If α <<< 5%, 1 - α \[\ce{\simeq}\] 1]

`"K"_"a" = (0.01 xx 0.02 xx 0.02)/1 = 4 xx 10^-6`