2021 ElectrochemistryConductance of Electrolytic Solutions – Ostwald's Dilution Hard Question 20 of 74 in Electrochemistry

The molar conductivity of 0.007 M acetic acid is $20\ S\ cm^2\ mol^{-1}$. What is the dissociation constant of acetic acid? Choose the correct option. ($\Lambda^\circ_{H^+}=350\ S\ cm^2mol^{-1}$, $\Lambda^\circ_{CH_3COO^-}=50\ S\ cm^2mol^{-1}$)

A $2.50\times10^{-4}\ mol\ L^{-1}$
B $1.75\times10^{-5}\ mol\ L^{-1}$
C $2.50\times10^{-5}\ mol\ L^{-1}$
D $1.75\times10^{-4}\ mol\ L^{-1}$

The answer is B.

Solution

$\Lambda^\circ_{m(CH_3COOH)} = \Lambda^\circ_{H^+} + \Lambda^\circ_{CH_3COO^-} = 350 + 50 = 400\ S\ cm^2\ mol^{-1}$ $\alpha = \frac{\Lambda_m^c}{\Lambda_m^\circ} = \frac{20}{400} = 5 \times 10^{-2}$ $K_a = C\alpha^2 = 0.007 \times (5 \times 10^{-2})^2 = 1.75 \times 10^{-5}$ mol $L^{-1}$

Concept dissociation constant from molar conductivity

Electrochemistry in past papers

74 questions from this chapter have appeared across 27 exam years.