Discussion
The limiting molar conductance for aqueous solution of CaCl2 at 298K is 271.6 S cm2 mol-1. If the limiting ionic conductance of Ca2+ ion at the same temperature is 119 S cm2 mol-1 what is the limiting ionic conductance of Cl- ion?
Answer: B
The limiting ionic conductance of Cl- is 76.3 S cm2 mol-1.
By Kohlrausch's law, the limiting molar conductance of a salt is the sum of the limiting conductances of its ions, each multiplied by the number of those ions in the formula. CaCl2 gives one Ca2+ and two Cl-:
\[ \begin{aligned} \Lambda^{\circ}_{m} &= \lambda^{\circ}_{\text{Ca}^{2+}} + 2\lambda^{\circ}_{\text{Cl}^{-}} \\ 271.6 &= 119 + 2\lambda^{\circ}_{\text{Cl}^{-}} \\ \lambda^{\circ}_{\text{Cl}^{-}} &= 152.6 / 2 = 76.3 \end{aligned} \]
152.6 is the answer you get by forgetting that there are two chloride ions per formula unit.
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