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JEE Main 2013 Paper

Section: Chemistry

Question : 11 of 90

Marks: +1, -0

$\mathrm{Ni}^{2+} < \mathrm{Co}^{2+} < \mathrm{Fe}^{2+} < \mathrm{Mn}^{2+}$ : ionic size
$\mathrm{Co}^{3+} < \mathrm{Fe}^{3+} < \mathrm{Cr}^{3+} < \mathrm{Sc}^{3+}$ : stability in aqueous solution
$\mathrm{Sc} < \mathrm{Ti} < \mathrm{Cr} < \mathrm{Mn}$ : number of oxidation states
$\mathrm{V}^{2+} < \mathrm{Cr}^{2+} < \mathrm{Mn}^{2+} < \mathrm{Fe}^{2+}$ : paramagnetic behaviour

Solution:

The exothermic hydration enthalpies of the given trivalent cations are:

\mathrm{Sc}^{+3} = 3960\,\text{kJ/mol}

\mathrm{Fe}^{+3} = 4429\,\text{kJ/}

mole

\mathrm{Co}^{+3} = 4653\,\text{kJ/mol}

\mathrm{Cr}^{+3} = 4563\,\text{kJ/mol}

Hence

\mathrm{Sc}^{+3}

is least hydrated; so least stable (not most stable)

\mathrm{Fe}^{+2}

contains 4 unpaired electrons where as

\mathrm{Mn}^{+2}

contains 5 unpaired electrons hence (4) is incorrect.

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