Q1 · CSIR-NET Chemistry, December 2013

Paper: CSIR-NET December 2013 · Subject: Inorganic Chemistry · Chapter: Acids and Bases · Topic: Acid and 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
Explanation
A hexaaqua ion is a stronger acid when the metal is small and highly charged. pKa values: [Fe(H₂O)₆]²⁺ ≈ 9.5, [Al(H₂O)₆]³⁺ ≈ 5.0, [Fe(H₂O)₆]³⁺ ≈ 2.2. Increasing acidity: Fe²⁺ < Al³⁺ < Fe³⁺.

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