Q38 · CSIR-NET Chemistry, December 2013
Paper: CSIR-NET December 2013 · Subject: Inorganic Chemistry · Chapter: Acids and Bases · Topic: Acids & Bases – General · Marks: 2 · Difficulty: Easy
Among the following, the correct acid strength trend is represented by
(a)$\left[\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}<\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}<\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$
(b)$\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}<\left[\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}<\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$
(c)$\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}<\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}<\left[\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}$
(d)$\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}<\left[\mathrm{Al}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}<\left[\mathrm{Fe}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}$
Answer
Answer: D ✓ checked by 4AB · confidence high
The source book printed A; on checking, D is correct — see the explanation.
Printed A; the order is reversed.
Explanation
Acidity of aqua ions rises with charge and with charge/size ratio: [Fe(H₂O)₆]²⁺ (pKa 9.5) < [Al(H₂O)₆]³⁺ (5.0) < [Fe(H₂O)₆]³⁺ (2.2).
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