Kohlrausch's Law

Kohlrausch's law of independent migration of ions states that the limiting molar conductivity of an electrolyte is the sum of the independent contributions of its constituent cation and anion. For acetic acid the required ions are H^+ and CH_3COO^-. Adding the limiting conductivities of HCl (H^+ + Cl^-) and sodium acetate (Na^+ + CH_3COO^-) gives H^+ + Cl^- + Na^+ + CH_3COO^-. Subtracting sodium chloride (Na^+ + Cl^-) cancels the Na^+ and Cl^- contributions, leaving exactly H^+ + CH_3COO^-, the value for acetic acid. The option subtracting only NaCl from one salt does not assemble the correct ion pair. Adding all three double-counts ions and gives no meaningful result. The claim that it cannot be obtained indirectly contradicts the very purpose of Kohlrausch's law, which is essential for weak electrolytes whose conductivity cannot be measured directly at infinite dilution. Examiners frequently test whether a student can connect ionic migration with the underlying principle rather than merely recalling an isolated fact. It is worth emphasising that this is not a special case but a representative example of how kohlrausch's law operates throughout redox reactions and electrochemistry. Plausibility check: the ion bookkeeping cancels Na^+ and Cl^- cleanly, leaving precisely the ions of acetic acid, confirming the method.