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Question:

Which of the following arrangements does not represent the correct order of the property stated against it?

Co3+<Fe3+<Cr3+<Sc3+: Stability in aqueous solution

Ni2+<Co2+<Fe2+<Mn2+: Ionic size

Sc<Ti<Cr<Mn: Number of oxidation states

V2+<Cr2+<Mn2+<Fe2+: Paramagnetic behaviour

Solution:

The exothermic hydration enthalpies of the given trivalent cations are: Sc+3=3960kJ/mole, Fe+3=4429kJ/mole, Co+3=4653kJ/mole, Cr+3=4563kJ/mole. Hence, Sc+3 is least hydrated, i.e., least stable (not most stable). Fe+2 contains 4 unpaired electrons whereas Mn+2 contains 5 unpaired electrons. Therefore, the correct order should be Mn2+>Fe2+>Co2+>Ni2+ for paramagnetic behavior.