Question

Which one of the following is correct with respect to molar ionic conductance of ions in aqueous solution?
(A) $L{i^ + } < N{a^ + } < {K^ + } < R{b^ + }$
(B) $R{b^ + } > {K^ + } > N{a^ + } > L{i^ + }$
(C) $S{r^{2 + }} > C{a^{2 + }} > M{g^{2 + }} > B{e^{2 + }}$
(D) $N{a^ + } > {K^ + } > L{i^ + } > R{b^ + }$

Answer 1

Hint: The conductance of an ion depends on its size in an aqueous medium or in the solvent. Different solutions may contain different concentrations and hence may contain different numbers of ions.

Complete answer:
The molar conductivity of an electrolyte is defined as the electrolytic conductivity divided by the molar concentration $C$ of the dissolved electrolyte.
$ \wedge = \dfrac{K}{C}\;\;\;{\text{or}}\;\;\; \wedge = KV$
${\text{S}}{\text{.I}}{\text{.}}$ of electrolytic conductivity is Siemens per meter $\left( {S{m^{ - 1}}} \right)$ or $Se{m^{ - 1}}$. ${\text{S}}{\text{.I}}{\text{.}}$ unit of molar conductivity is Siemens square meter per mole $\left( {{\text{S}}{{\text{m}}^{\text{2}}}{\text{mo}}{{\text{l}}^{{\text{ - 2}}}}} \right)$ or ${\text{sc}}{{\text{m}}^{\text{2}}}{\text{mo}}{{\text{l}}^{{\text{ - 1}}}}$
Ionic conductivity depends on size in aqueous solution. Bigger is the ionic size and less is its conductance.
Example: The order of size of hydrated ionic radii of alkali metal cation is as-
$L{i^ + } > N{a^ + } > {K^ + } > R{b^ + } > C{S^ + }$
So, The correct answer is (A).

Additional Information: The electrolytic conductivity depends on the number of ions present in a unit volume of a solution. On dilution the degree of dissociation increases, the number of conducting ions in the solution also increases. But, the number of conducting ions per unit volume decreases. Hence the activity of ions decreases and the electrolytic conductivity also decreases.
For a strong electrolyte, the electrolytic conductivity increases sharply with an increase in concentration. Electrolytic conductivity is very low in dilute solutions and increases much more gradually with increase in the concentration which is for the weak electrolyte and in the solution, this increase is due to increases in active ions.

Note:
In $1880$ the German physicist George Kohlrausch introduced the concept of molar conductivity which is used to compare conductance of different solutions.