Question

Which of the following statements is incorrect ?

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> AIIMS 2014

• A. The oxidation state of $K$ in $K{O}_2$ is $+1$
• B. $MgS{O}_4$ is readily soluble in water
• C. $Na$ is used to make a $Na/Pb$ alloy
• D. $L{i}^{+}$ has minimum degree of hydration

Answer 1

The correct option is D $L{i}^{+}$ has minimum degree of hydration

$K{O}_2$ is a superoxide. In all superoxides, oxygen has an oxidation state of − 1/2.
Hence, oxidation state of $K$ in $K{O}_2$ is +1.

With the increase in the cationic size on moving down the group, the extent of the hydration of the cation decreases. Also the lattice energy decrease down the group.
For a substance to dissolve, the hydration energy must exceed the lattice energy.

For group II compounds (sulphates ), on descending the group, the hydration energy decreases more rapidly than the lattice energy. Thus, the compounds becomes less soluble as the metal gets larger.
Hence, the solubility of the alkaline earth's metal sulphates in water decreases on moving down the group. Thus, $MgS{O}_4$ is readily soluble in water.

$Na$ is used to make a $Na/Pb$ alloy

The hydration enthalpies of alkali metal ions decreases with increase in ionic sizes Hence, the order is $L{i}^{+}>N{a}^{+}>K^{+}>R{b}^{+}>C{s}^{+}$. Therefore, $L{i}^{+}$ has maximum degree of hydration.

Hence, option (a) is correct answer.