MCAT General Chemistry Question 24: Answer and Explanation

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Question: 24

9. When dissolved in water, which of the following ions is most likely to form a complex ion with H₂O?

A. Na⁺
B. Fe²⁺
C. Cl^–
D. S^2–

Correct Answer: B

Explanation:

Iron is a transition metal. Transition metals can often form more than one ion. Iron, for example, can be Fe²⁺ or Fe³⁺. The transition metals, in these various oxidation states, can often form hydration complexes with water. Part of the significance of these complexes is that, when a transition metal can form a complex, its solubility within the related solvent will increase. The other ions given might dissolve readily in water, but because none of them are transition metals, they will not likely form complexes.

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