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Practice Problems for the CHM112 final exam.

These problems do not include everything that might be on the final, but certainly are indicative of the level of difficulty.


This set of problems considers the chemistry of manganese. The following set of data will be helpful:

species Hof (kJ/mol) So (J/mol•K) Gof (kJ/mol)  
Mn(OH)2(s) –693.7     Ksp = 2×10–13
Mn2+(aq) –220.8 –73.6 –228.1  
MnOH+(aq)       Kb = 3.6×10–4
H2O(l) –285.8 69.9 –237.2  
H+(aq) 0.0 0.0 0.0  
H3O+(aq) –285.8 69.9 –237.2  
OH–(aq) –230.0 –10.8 –157.2  
Mn2+(aq) + 2e–Mn(s) Eored = –1.182 V
2H2O(l) + 2e– H2(g) + 2OH–(aq) Eored = –0.828 V


The goal of the first ten questions is to compare the solubility of manganese(II) hydroxide at 25 oC and 60 oC.

1. Calculate the molar solubility of Mn(OH)2 at 25 oC.

2. Calculate the pH of a saturated solution of Mn(OH)2 at 25 oC.

3. Calculate Go for the solubilization reaction of Mn(OH)2 at 25 oC.

4. Calculate Gof for Mn(OH)2 at 25 oC.

5. Calculate Ho for the solubilization reaction of Mn(OH)2 at 25 oC.

6. Calculate So for the solubilization reaction of Mn(OH)2 at 25 oC.

7. Calculate So for Mn(OH)2 at 25 oC.

8. Calculate Go for the solubilization reaction of Mn(OH)2 at 60 oC.

9. Calculate Ksp for Mn(OH)2 at 60 oC.

10. Calculate the molar solubility of Mn(OH)2 at 60 oC.
 
 

The next eight questions look at the solubility of manganese(II) hydroxide at 60 oC in terms of pH.

11. Calculate Ho for the autoionization of water at 25 oC.

12. Calculate So for the autoionization of water at 25 oC.

13. Calculate Go for the autoionization of water at 25 oC.

14. Calculate Go for the autoionization of water at 60 oC.

15. Calculate Kw at 60 oC.

16. Calculate [H3O+]e and [OH–]e of pure water at 60 oC.

17. Calculate the pH of pure water at 60 oC.

18. Calculate the pH of a saturated solution of Mn(OH)2 at 60 oC.
 

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 oC. (Hint: use the rule of multiple equilibria.)

20. Calculate the equilibrium concentration of MnOH+(aq) at 25 oC. 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 oC 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 oC.

24. Calculate the pH of a 0.10 M solution of Mn(NO3)2 at 25 oC.
 

The goal of the next seven questions is to find the standard reduction potential of manganese(II) in a basic environment.

25. Calculate Eo for Pt(s)|H2(g)|H+(aq)||Mn2+(aq)|Mn(s) at 25 oC.

26. Calculate Go for Pt(s)|H2(g)|H+(aq)||Mn2+(aq)|Mn(s) at 25 oC.

27. Calculate Kc for Pt(s)|H2(g)|H+(aq)||Mn2+(aq)|Mn(s) at 25 oC.

28. Calculate Kc for Pt(s)|H2(g)|OH–(aq)||Mn(OH)2(s)|Mn(s) at 25 oC. (Use the rule of multiple equilibria.)

29. Calculate Go for Pt(s)|H2(g)|OH–(aq)||Mn(OH)2(s)|Mn(s) at 25 oC.

30. Calculate Eo for Pt(s)|H2(g)|OH–(aq)||Mn(OH)2(s)|Mn(s) at 25 oC.

31. Calculate the reduction potential for Mn(OH)2(s) + 2e–Mn(s) + 2OH–(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 oC 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?