Question

Select the correct order of ionic conductance of the following cations in an aqueous solution.

• A. ${Li}^{+}<{Na}^{+}<K^{+}<{Rb}^{+}$
• B. ${Li}^{+}>{Na}^{+}>K^{+}>{Rb}^{+}$
• C. ${Li}^{+}<{Na}^{+}>K^{+}>{Rb}^{+}$
• D. ${Li}^{+}={Na}^{+}<K^{+}<{Rb}^{+}$

Answer 1

The correct option is A ${Li}^{+}<{Na}^{+}<K^{+}<{Rb}^{+}$

Smaller is the size of cation, greater will be the mobility and hence, the conductivity. The size of the cation increases as we move from $L{i}^{+}$ to $R{b}^{+}$. Hence, the conductivity should decrease. However, in aqueous solution, due to very small size of $L{i}^{+}$, it is strongly hydrated and hence, its effective hydrated size is greater than that of $R{b}^{+}$.
So, the correct order of increasing ionic conductance is as follows:

${Li}^{+}<{Na}^{+}<K^{+}<{Rb}^{+}$
Hence, option A is correct.