Exam #4 Key

CHEM 121Spring 2007Dr. FritschExam #4 v1 KEY100 pointsName_______________________________Instructions:1. Work carefully and consider all the possible answers in the multiple choice portion.2. In the short answer portion, show your work. I will award partial credit.3. Check your work. Does you answer make sense?K aHCN 6.2 x 10 -10HOCl 2.9 x 10 -8HNO 2 7.2 x 10 -4CH 3 COOH 1.8 x 10 -5NH 4+5.56 x 10 -101

Percentage (%)HomeworkExamsQuizzesOverallThe scores above reflect the lowest quiz score dropped and thelowest exam counted at ½ credit.2

Multiple choice section (4 pts each)1. Which response includes all the following processes that are accompanied by an increase inentropy?A. I, II, III, IVB. I and IIC. II, III, IVD. III and IVE. I and IVI. 2SO 2 (g) + O 2 (g) SO 3 (g)II. H 2 O(l) H 2 O(s)III. Br 2 (l) Br 2 (g)IV. H 2 O 2 (l) H 2 O(l) + (1/2)O 2 (g)Answer:____D______2. Which one of the following is a buffer solution?A. 0.40 M HCN and 0.10 KCNB. 0.20 M CH 3 COOHC. 1.0 M HNO 3 and 1.0 M NaNO 3D. 0.10 M KCNE. 0.50 M HCl and 0.10 NaClAnswer:_____A_____3. Over what range of pH is a HOCl NaOCl buffer effective?A. pH 2.0 – pH 4.0B. pH 7.5 – pH 9.5C. pH 6.5 – pH 8.5D. pH 6.5 – pH 9.5E. pH 1.0 – pH 14.0Answer:____C______3

4. On the basis of the information above, a buffer with a pH = 9 can best be made by usingAcidH 3 PO 4-H 2 PO 4HPO 42-K a7 10 38 10 85 10 13A. pure NaH 2 PO 4 .B. H 3 PO 4 + H 2 PO 42-C. H 2 PO 4 - + PO 4 3- .D. H 2 PO 4 - + HPO 4 2- .E. HPO 4 2- + PO 4 3- .Answer:____D______5. For which type of titration will the pH be basic at the equivalence point?A. Strong acid vs. strong base.B. Strong acid vs. weak base.C. Weak acid vs. strong base.D. All of the above.E. None of the above.Answer:____C______6. The normal freezing point of ammonia is 78C. Predict the signs of H, S, and G forammonia when it freezes at 80C and 1 atm: NH 3 (l) NH 3 (s)H S GA. - - 0B. - + -C. + - +D. + + 0E. - - -Answer:____E______4

7. Even though, your calculations suggest a reaction should be spontaneous (G rxn

Short answer questions. Show your work for partial credit.9. (15 pts) You are asked to go into the lab and prepare 1.0 L of an 0.5 M acetic acid - sodiumacetate buffer solution with a pH of 4.00 ± 0.02. What masses of CH 3 COOH (60 g/mol) andCH 3 COONa (82 g/mol) should be used?6

10. (13 pts) Consider the reactions below. Classify each one with the following system byplacing a number in the box to the left of each reaction. (Hint: You will use each number onlyonce.)1. Spontaneous at all temperatures2. Non-spontaneous at low temperatures, but spontaneous at high temperatures3. Spontaneous at low temperatures, but non-spontaneous at high temperatures4. Non-spontaneous at all temperaturesClassificationH rxn (kJ/mol)2 H 2 (g) 2H(g) +436.41 2CH 3 OH(l) + 3O 2 (g) 2CO 2 (g) + 4H 2 O(g) -1452.84 SO 2 (g) + Cl 2 (g) SO 2 Cl 2 (g) +97.33 N 2 (g) + 3H 2 (g) 2NH 3 (g) -92.611. (12 pts) Consider the reactions below. Circle the Lewis base in the reactants of eachreaction.Ag + + 2 NH 3 H 3 N-Ag-NH 3+Cl - + AlCl 3 AlCl 4-H 2 SO 4 + OH - HSO 4 - + H 2 O7

12. Consider the titration of 20.00 mL of 0.215 M NH 3 (K b = 1.8 10 5 ) with 0.153 M HCl.NH 3 (aq) + HCl(aq) NH 4 Cl(aq)a. (7 pts) Calculate the pH after 13.21 mL of HCl have been added.b. (7 pts) Calculate the volume of HCl added at the equivalence point.8

c. (7 pts) Calculate the pH at the equivalence point for this titration.d. (7 pts) Calculate the pH after 35.00 mL of HCl have been added.9

Extra credit (6 pts)Answer the three following questions about how the approximation method would be used todetermine equilibrium concentrations for the reactants and products in general acid ionizationreaction below.HA + H 2 O A - + H 3 O +1. Explain the rationale for why the approximation method is justified in determiningequilibrium concentrations.The change in concentration of the weak acid or base will be small in comparison to thestarting concentration.2. What signals do you look for a clue that it might be appropriate in a given case?High concentrations (>0.1 M)and low K values (