JEE Main · 2021 · 17 Mar 2021 · Shift 1 · Equilibrium

0.01 moles of a weak acid

HA (Ka = 2.0 \times 1 0^{- 6})

is dissolved in 1.0 L of 0.1 M HCl solution. The degree of dissociation of HA is ____

\times 1 0^{- 5}

.
(Round off to the nearest integer).
[ Neglect volume change on adding HA.
Assume degree of dissociation <<1]

Answer: 2

Solution

Hydrochloric acid is completely dissociated, so

[H^{+}] = 0.1

M and the trace contributed by HA is negligible beside it. With

α ≪ 1

the undissociated acid stays at its initial concentration

0.01

M while

[A^{-}] = 0.01 α

, so

K_{a} = \frac{[ H ^{+} ] [ A ^{-} ]}{[ HA ]} = \frac{0.1 \times 0.01 α}{0.01} = 0.1 α

Hence

α = \frac{2.0 \times 1 0 ^{- 6}}{0.1} = 2.0 \times 1 0^{- 5}

, so the degree of dissociation of HA is

2 \times 1 0^{- 5}

JEE Main · 2021 · 17 Mar 2021 · Shift 1 · Integer answer