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Question

The mobilities of the alkali metal ions in aqueous solution are $$Li^+ < Na^+ < K^+ < Rb^+ < Cs^+$$ because :


A
greater is the degree of hydration, lesser is the mobility in aqueous medium
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B
larger the size of cation, greater is the mobility in aqueous medium
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C
larger the size of cation, lesser is the mobility in aqueous medium
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D
lesser the degree of hydration, lesser is the mobility of ions in aqueous medium.
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Solution

The correct option is A greater is the degree of hydration, lesser is the mobility in aqueous medium
small size cations such as Lithium, sodium are heavily hydrated i.e. more no. of water molecules are surrounded to small cation due to which the hydrated radii of small size cation is large hence to degree of hydration is greater. If the degree of hydration is large, the hydrated cation will move less freely i.e. low ionic mobility in aqueous medium. On going down the group in alkali metals, the ionic mobility increases as less no. of water molecules are surrounded to large cations. Hence, small size cation, high degree of hydration, low ionic mobility. large size cation, low degree of hydration, high ionic mobility.
The order of ionic mobilities of alkali metal ions in aqueous solution are
$${ Li }^{ + }\quad <\quad { Na }^{ + }\quad <\quad { K }^{ + }\quad <\quad { Rb }^{ + }\quad <\quad { Cs }^{ + }$$

Chemistry

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