Question

Analysis of a solution of Co₂(SO₄)₃ establishes that the concentration of SO₄²⁻ is 0.060 mol L⁻¹. What is the concentration of Co³⁺ in this solution?

• A. 0.010 mol L⁻¹
• B. 0.030 mol L⁻¹
• C. 0.040 mol L⁻¹
• D. 0.060 mol L⁻¹

Answer 1

Answer: (C)

0.040 mol L⁻¹

Answer 2

The salt cobalt(III) sulfate, Co₂(SO₄)₃, dissociates in water according to the following ionic equation: $\text{Co}_2\text{(SO}_4\text{)}_3\text{(s)} \rightarrow 2\text{Co}^{3+}\text{(aq)} + 3\text{SO}_4^{2-}\text{(aq)}$ This stoichiometry shows that for every 1 mole of Co₂(SO₄)₃ that dissolves, 2 moles of Co³⁺ ions and 3 moles of SO₄²⁻ ions are produced. Therefore, the molar ratio of Co³⁺ ions to SO₄²⁻ ions in the solution is 2:3.

Given the concentration of SO₄²⁻ is 0.060 mol L⁻¹. Let the concentration of Co³⁺ be $C_{\text{Co}^{3+}}$. Using the stoichiometric ratio: $\frac{[\text{Co}^{3+}]}{[\text{SO}_4^{2-}]} = \frac{2}{3}$ $[\text{Co}^{3+}] = \frac{2}{3} \times [\text{SO}_4^{2-}]$ $[\text{Co}^{3+}] = \frac{2}{3} \times 0.060 \text{ mol L}^{-1}$ $[\text{Co}^{3+}] = 0.040 \text{ mol L}^{-1}$