The molar conductivities at infinite dilution of sodium acetate, hydrochloric acid and sodium chloride are 91.0, 425.9 and 126.4 S cm2 mol−1 respectively at 298 K. Calculate the molar conductivity of acetic acid at infinite dilution.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 390.5 S cm2 mol−1
By Kohlrausch’s law of independent migration of ions, the limiting molar conductivity of an electrolyte is the sum of the contributions of its cation and anion. Adding the values for \( \ce{CH3COONa} \) and \( \ce{HCl} \) and subtracting that for \( \ce{NaCl} \) leaves only \( \ce{H^+} \) and \( \ce{CH3COO^-} \):
\( \displaystyle \Lambda^{\circ}_{m}(\ce{CH3COOH})=\Lambda^{\circ}_{m}(\ce{CH3COONa})+\Lambda^{\circ}_{m}(\ce{HCl})-\Lambda^{\circ}_{m}(\ce{NaCl}) \)
\( \displaystyle \Lambda^{\circ}_{m}(\ce{CH3COOH})=(91.0+425.9-126.4)\,\mathrm{S\,cm^{2}\,mol^{-1}} \)
\( \displaystyle \Lambda^{\circ}_{m}(\ce{CH3COOH})=390.5\,\mathrm{S\,cm^{2}\,mol^{-1}} \)