The mobilities of the alkali metal ions in aqueous solution are `Li^(+) lt Na^(+) lt K^(+) lt Rb^(+) lt Cs^(+) ` because

Explanation

Smaller the size of alkali metal ion, greater is its degree of hydration and lesser is its mobility in aqueous solution due to greater mass. Since the size of alkali metal ions follows the order `Li^(+) lt Na^(+) lt K^(+) lt Rb^(+) lt Cs^(+)`, the ionic mobility of alkali metal ions is in the order
`Li^(+) lt Na^(+) lt K^(+) lt Rb^(+) lt Cs^(+)`.