Question

Ionic mobility of which of the following alkali metal ions is lowest when aqueous solution of their salts are put under an electric field ?

• A. K
• B. Rb
• C. Li
• D. Na

Answer 1

The Correct Option is C

The question pertains to the ionic mobility of alkali metal ions in aqueous solution under an electric field. Ionic mobility is a measure of how quickly an ion moves through a solvent when subjected to an electric field. It depends on the size of the hydrated ion; smaller hydrated ions generally have lower mobility due to increased interactions with the solvent molecules.

Alkali metal ions like lithium (Li⁺), sodium (Na⁺), potassium (K⁺), and rubidium (Rb⁺) differ in their hydration radii, which affect their ionic mobility. Here’s the step-by-step reasoning:

1. The ionic radius in the gaseous state increases in the order: Li⁺ < Na⁺ < K⁺ < Rb⁺. This is due to the increase in the number of electron shells.
2. Upon hydration, however, the situation changes as smaller ions tend to have a larger hydration shell. Therefore, the effective size of the hydrated ion is different from the gaseous state.
3. Lithium ion (Li⁺), being the smallest cation in its gaseous form, strongly attracts water molecules, resulting in a relatively large hydrated radius.
4. In contrast, larger ions like K⁺ and Rb⁺ have smaller hydration shells, thus larger ionic mobility.
5. Therefore, the ionic mobility decreases in the order: Rb⁺ > K⁺ > Na⁺ > Li⁺.

Consequently, the alkali metal ion with the lowest ionic mobility when diluted in an aqueous solution and subjected to an electric field is Lithium (Li⁺).

Hence, the correct answer is: Li