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

Options

A. K
B. Rb
C. Li
D. Na

Answer

C

Explanation

The hydration enthalpy of alkali metal ions decreases with increase in ionic sizes i.e.,
`Li^(+) gt Na^(+) gt K^(+) gt Rb^(+) gt Cs^(+)`
Hence, lithium having maximum degree of hydration will be least mobile.
The order of ionic mobility is `[Li_((aq))]^(+) lt [Na_((aq))]^(+) lt [K_((aq))]^(+) lt [Rb_((aq))]^(+)`