sblock Elements 2 Question 1
11. The alkaline earth metal nitrate that does not crystallise with water molecules, is
(2019 Main, 9 Jan I)
(a) $\mathrm{Ca}\left(\mathrm{NO}_3\right)_2$
(b) $\mathrm{Sr}\left(\mathrm{NO}_3\right)_2$
(c) $\mathrm{Ba}\left(\mathrm{NO}_3\right)_2$
(d) $\mathrm{Mg}\left(\mathrm{NO}_3\right)_2$
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Answer:
Correct Answer: 11. (c)
Solution: A saturated aqueous solution of $\stackrel{1}{M}\left(\mathrm{NO}_3\right)_2$ on crystallisation will produce hydrated crystal, $\stackrel{\text { II }}{M}\left(\mathrm{NO}_3\right)_2 \cdot n \mathrm{H}2 \mathrm{O}$ only when hydration enthalpy ( $\Delta H^{\circ}{ }{\text {hyd }}$ ) of $M^{2+}$ ion will be appreciably more negative.
Hydration of an ion depends on its size. Smaller the size of an ion, higher will be its charge density and as a result it will remain more solvated (hydrated) through ion dipole interaction. Size of group- 2 metal ions increases on going down the group. So, their ability to form hydrated crystals follows the order:
$$ \mathrm{Be}^{2+} \gg \mathrm{Mg}^{2+} \gg \mathrm{Ca}^{2+} \gg \mathrm{Sr}^{2+} \gg \mathrm{Ba}^{2+} $$
Thus, $\mathrm{Ba}\left(\mathrm{NO}_3\right)_2$ is slightly or almost insoluble in water.
BETA
