Geo-Scientist Prelims 2020 Chemistry Q30: Which one of the following represents the correct order of acid…

Answer: (b) [Fe(H₂O)₆]²⁺ < [Fe(H₂O)₆]³⁺ < [Al(H₂O)₆]³⁺

Acidity of [M(H₂O)₆]ⁿ⁺ rises with charge density of Mⁿ⁺: Fe²⁺ is weakest. On simple charge/size (Al³⁺ 53.5 pm < Fe³⁺ 64.5 pm) the key ranks Fe²⁺ < Fe³⁺ < Al³⁺ → (b). Note: measured pKa values (Fe³⁺ ≈ 2.2, Al³⁺ ≈ 5.0) actually make Fe³⁺ the stronger acid, i.e. order (a).