General characteristic chemical properties of the first set of transition elements, titanium to copper — practice questions

(i) The equation for the equilibrium is: $[Co(H_2O)_6]^{2+}(aq) + 4Cl^-(aq) \rightleftharpoons [CoCl_4]^{2-}(aq) + 6H_2O(l)$ (ii) The product complex is $[CoCl_4]^{2-}$. The coordination number is 4 (four chloride ligands). The shape is tetrahedral. (iii) This is a ligand substitution reaction because the six water ($H_2O$) ligands originally bonded to the $Co^{2+}$ ion have been replaced (substituted) by four chloride ($Cl^-$) ligands.

Step 1: Determine the stoichiometry. The relevant half-equations are: $MnO_4^-(aq) + 8H^+(aq) + 5e^- \rightarrow Mn^{2+}(aq) + 4H_2O(l)$ $Fe^{2+}(aq) \rightarrow Fe^{3+}(aq) + e^-$ To balance electrons, the molar ratio of $MnO_4^-$ to $Fe^{2+}$ is 1:5.

Step 2: Calculate the moles of $MnO_4^-$ used. Moles = concentration × volume Moles $MnO_4^- = 0.0200 \text{ mol dm}^{-3} \times (22.50 / 1000) \text{ dm}^3 = 4.50 \times 10^{-4} \text{ mol}$.

Step 3: Calculate the moles of $Fe^{2+}$ in the sample. From the 1:5 ratio, Moles $Fe^{2+} = 5 \times$ Moles $MnO_4^- = 5 \times 4.50 \times 10^{-4} = 2.25 \times 10^{-3} \text{ mol}$.

Step 4: Calculate the concentration of $Fe^{2+}$. Concentration = moles / volume Concentration $Fe^{2+} = 2.25 \times 10^{-3} \text{ mol} / (25.0 / 1000) \text{ dm}^3 = 0.0900 \text{ mol dm}^{-3}$.