QRC Chemist Evaluation Practice Exam 2025

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What are the geometric shape and oxidation number of the copper atom in the complex ion [Cu(NH3)4(H2O)2]²⁺?

Square planar; +1

Square planar; +2

In the complex ion [Cu(NH3)4(H2O)2]²⁺, the central copper atom is surrounded by four ammonia molecules (NH3) and two water molecules (H2O). To determine the geometric shape of this complex, it is important to note that it contains six coordinate bonds: four from the NH3 ligands and two from the H2O ligands. With six ligands around the central metal atom, the geometry that optimally accommodates this coordination number is octahedral.

Regarding the oxidation state of copper in this complex, we begin by considering the overall charge of the complex ion, which is +2. The ammonia and water molecules are neutral ligands, contributing no charge. Thus, the oxidation state of copper must be +2 in order to balance the overall +2 charge of the complex.

Putting these pieces of information together, we find that the copper atom in [Cu(NH3)4(H2O)2]²⁺ has an octahedral geometry and an oxidation number of +2, making option B the correct choice.

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Tetrahedral; +1

Octahedral; +2

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