The goal of the first ten questions is to compare the solubility of manganese(II) hydroxide at 25 °C and 60 °C.
1. Calculate the molar solubility of Mn(OH)2 at 25 °C.
2. Calculate the pH of a saturated solution of Mn(OH)2 at 25 °C.
3. Calculate ΔG° for the solubilization reaction of Mn(OH)2 at 25 °C.
4. Calculate ΔG°f for Mn(OH)2 at 25 °C.
5. Calculate ΔH° for the solubilization reaction of Mn(OH)2 at 25 °C. ΔH°f(Mn(OH)2(s)) = –693.7 kJ/mol
6. Calculate ΔS° for the solubilization reaction of Mn(OH)2 at 25 °C.
7. Calculate S° for Mn(OH)2 at 25 °C.
8. Calculate ΔG° for the solubilization reaction of Mn(OH)2 at 60 °C.
9. Calculate Ksp for Mn(OH)2 at 60 °C.
10. Calculate the molar solubility of Mn(OH)2 at 60 °C.
The next eight questions look at the solubility of manganese(II) hydroxide at 60 °C in terms of pH.
11. Calculate ΔH° for the autoionization of water at 25 °C.
12. Calculate ΔS° for the autoionization of water at 25 °C.
13. Calculate ΔG° for the autoionization of water at 25 °C.
14. Calculate ΔG° for the autoionization of water at 60 °C.
15. Calculate Kw at 60 °C.
16. Calculate [H3O+]e and [OH–]e of pure water at 60 °C.
17. Calculate the pH of pure water at 60 °C.
18. Calculate the pH of a saturated solution of Mn(OH)2 at 60 °C.
The next six questions look at the acid/base properties of manganese(II) and manganese(II) hydroxide.
19. Calculate Kb for Mn(OH)2 at 25 °C. (Hint: use the rule of multiple equilibria. Kb for MnOH+ = 3.6×10–4)
20. Calculate the equilibrium concentration of MnOH+(aq) at 25 °C. Use the usual assumption that only the first ionization is important for a weak base.
21. Calculate the [OH–]e and pH of a saturated Mn(OH)2 solution at 25 °C using Kb(Mn(OH)2) and Kb(MnOH+).
22. The pH values calculated from question 2 and question 21 should be the same; why are they different?
23. Calculate Ka for Mn2+(aq) at 25 °C.
24. Calculate the pH of a 0.10 M solution of Mn(NO3)2 at 25 °C.
The goal of the next seven questions is to find the standard reduction potential of manganese(II) in a basic environment.
25. Calculate E° for Pt(s) | H2(g) | H+(aq) || Mn2+(aq) | Mn(s) at 25 °C.
26. Calculate ΔG° for Pt(s) | H2(g) | H+(aq) || Mn2+(aq) | Mn(s) at 25 °C.
27. Calculate Kc for Pt(s) | H2(g) | H+(aq) || Mn2+(aq) | Mn(s) at 25 °C.
28. Calculate Kc for Pt(s) | H2(g) | OH–(aq) || Mn(OH)2(s) | Mn(s) at 25 °C. (Use the rule of multiple equilibria.)
29. Calculate ΔG° for Pt(s) | H2(g) | OH–(aq) || Mn(OH)2(s) | Mn(s) at 25 °C.
30. Calculate E° for Pt(s) | H2(g) | OH–(aq) || Mn(OH)2(s) | Mn(s) at 25 °C.
31. Calculate the reduction potential for Mn(OH)2(s) + 2e– → Mn(s) + 2 OH–(aq).
Some thought questions:
32. A common misconception amongst beginning chemistry students is that the pH of a solution tells you if an acid is strong or weak. Why is this statement false?
33. It can be argued that the Ka of pure water at 25 °C is 1.8×10–16. Show how this value is arrived at.
34. The concentration of a solid does not change as a function of reaction progress and so it is not used in mass action expressions. Despite this, the surface area of a solid strongly affects the reaction rate. Explain.
35. The First Law of Thermodynamics says that energy is conserved. Why, then, is it impossible to make a perpetual motion machine?