Question

The solubility of alkali metal salts in water is due to the fact that the cations get hydrated by water molecules. What is the relative degree of hydration?

• A. $Cs_{\left(a q\right)}^{+}>Rb_{\left(a q\right)}^{+}>K_{\left(a q\right)}^{+}>Na_{\left(a q\right)}^{+}>Li_{\left(a q\right)}^{+}$
• B. $Li_{\left(a q\right)}^{+}>Na_{\left(a q\right)}^{+}>K_{\left(a q\right)}^{+}>Rb_{\left(a q\right)}^{+}>Cs_{\left(a q\right)}^{+}$
• C. $Na_{\left(a q\right)}^{+}>K_{\left(a q\right)}^{+}>Rb_{\left(a q\right)}^{+}>Cs_{\left(a q\right)}^{+}>Li_{\left(a q\right)}^{+}$
• D. $Cs_{\left(a q\right)}^{+}>Na_{\left(a q\right)}^{+}>Li_{\left(a q\right)}^{+}>K_{\left(a q\right)}^{+}>Rb_{\left(a q\right)}^{+}$

Answer 1

Answer: (B)

As hydration energy $ \propto $ charge on cation $ \propto \frac{1}{\text{size of cation}}$
As on moving down size of ion increases so degree of hydration decreases hence the correct order is
$Li_{\left(a q\right)}^{+}>Na_{\left(a q\right)}^{+}>K_{\left(a q\right)}^{+}>Rb_{\left(a q\right)}^{+}>Cs_{\left(a q\right)}^{+}$