Kohlrausch examined the limiting molar conductivity (Λ°m) of a number of strong electrolytes and observed certain regularities. On the basis of these observations he enunciated the Kohlrausch law of independent migration of ions. The law states that the limiting molar conductivity of an electrolyte can be represented as the sum of the individual contributions of the anion and the cation of the electrolyte. Thus, for sodium chloride:
\(\Lambda^{\circ}_m(\text{NaCl}) = \lambda^{\circ}_{\text{Na}^+} + \lambda^{\circ}_{\text{Cl}^-}\)
Using the Kohlrausch law, it is possible to calculate Λ°m for any electrolyte from the λ° of its individual ions. For a weak electrolyte such as acetic acid, Λ°m can also be obtained from the Λ°m values of strong electrolytes:
\(\Lambda^{\circ}_m(\text{CH}_3\text{COOH}) = \Lambda^{\circ}_m(\text{CH}_3\text{COONa}) + \Lambda^{\circ}_m(\text{HCl}) - \Lambda^{\circ}_m(\text{NaCl})\)
At 18°C, the limiting molar conductivities of H+ and CH3COO− ions are 315 and 35 S cm2 mol−1 respectively. What is the limiting molar conductivity of CH3COOH at 18°C (in S cm2 mol−1)?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 350
The passage states the Kohlrausch law of independent migration of ions: the limiting molar conductivity of an electrolyte is the sum of the contributions of its cation and its anion. Acetic acid gives \( \ce{H^+} \) and \( \ce{CH3COO^-} \) ions:
\( \displaystyle \Lambda^{\circ}_m(\ce{CH3COOH})=\lambda^{\circ}_{\ce{H^+}}+\lambda^{\circ}_{\ce{CH3COO^-}} \)
\( \displaystyle \Lambda^{\circ}_m(\ce{CH3COOH})=(315+35)\ \mathrm{S\,cm^2\,mol^{-1}} \)
\( \displaystyle \Lambda^{\circ}_m(\ce{CH3COOH})=350\ \mathrm{S\,cm^2\,mol^{-1}} \)