CHEMISTRY 204 Practice Hour Exam II. Dr. D. DeCoste T.A.

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1 CHEMISTRY 204 Practice Hour Exam II Spring 2019 Dr. D. DeCoste Name Signature T.A. Section This exam contains 23 questions on 8 numbered pages. Check now to make sure you have a complete exam. You have two hours to complete the exam. Determine the best answer to the first 20 questions and enter these on the special answer sheet. Also, circle your responses in this exam booklet. Show all of your work and provide complete answers to questions 21, 22 and (60 pts.) 21 (20 pts.) 22 (20 pts) 23 (20 pts.) Total (120 pts) Useful Information: Unless otherwise noted, all solutions referred to on this exam are aqueous solutions at 25 C. On this exam, H3O + and H + are used interchangeably. Kw = [H + ][OH - ] = 1.0 x at 25 C. For ax 2 +bx + c = 0, x = b ± b 2a 2 4ac ph = -log[h + ] Ka = Here are some Ka values you may need: H2C2O4: Ka1 = 6.5 x 10-2, Ka2 = 6.1 x 10-5 HNO2: 4 x 10-4 HF: 7.2 x 10-4 HC2H3O2: 1.8 x 10-5 HOCl: 3.5 x 10-8 HCN: 6.2 x HOI: 2 x [H ] - K w + 2 [H ] - K [HA] o - + [H ] w And Kb values: NH3: 1.8 x 10-5 C6H5NH2: 3.8 x (C2H5)3N: 4 x 10-4

2 PracticeHour Exam II Page No Which of the following correctly ranks the species from strongest to weakest base? a) OI - > CN - > NH3 > C2H3O2 - > F - > NO3 - > H2O b) CN - > OI - > NH3 > C2H3O2 - > F - > H2O > NO3 - c) NH3 > CN - > OI - > C2H3O2 - > F - > NO3 - > H2O d) NH3 > OI - > CN - > C2H3O2 - > F - > NO3 - > H2O e) OI - > NH3 > CN - > C2H3O2 - > F - > H2O > NO3-2. The ph of a M solution of NaA is Identify the acid HA. a) HF b) HOCl c) HCl d) HCN e) HC2H3O2 3. You have two solutions, one of HF and one of HC2H3O2. You measure the ph values of each solution and find they are the same. Determine the ratio of [HC2H3O2]/[HF], assuming the initial concentrations of the acids are equal to their equilibrium concentrations. a) b) 40 c) 1.0 d) 0.10 e) None of these 4. Consider ml of a strong acid solution with a ph = What mass of KOH should you add to this solution to change the ph to 5.00? Assume the solid KOH does not appreciably change the volume of the solution. (1000 mg = 1 g, exactly) a) mg b) 5.55 mg c) 56.1 mg d) 56.1 g e) None of these 5. Consider a solution made by mixing two acid solutions such that the initial concentration of HF = 2.0 M and the initial concentration of HCN = 1.0 M. Determine [CN - ] for this solution. a) 6.2 x M b) 3.1 x M c) 1.6 x 10-8 M d) 2.5 x 10-5 M e) 7.2 x 10-4 M 6. Consider ml of a buffer solution solution of 2.00 M HF and 2.00 M NaF. Determine the change in ph when 2.00 x 10-3 mol of NaOH is added to the buffer. Assume the solid NaOH does not appreciably change the volume of the solution. a) The ph increases by b) The ph decreases by c) The ph increases by d) The ph decreases by e) The ph is the same when recorded to the hundredths place.

3 PracticeHour Exam II Page No Determine the ph of a 1.0 x 10-5 M solution of HCN. a) 4.52 b) 6.28 c) 6.77 d) 6.90 e) Consider the titration of ml of M H2A (Ka1 = 1.0 x 10-5, Ka2 = 1.0 x 10-9 ) with M NaOH. 8. Determine the ph before the titration begins. a) 1.00 b) 3.00 c) 3.50 d) 5.00 e) None of these. 9. Determine the ph after a total of 75.0 ml of M NaOH has been added. a) 5.00 b) 6.42 c) 7.00 d) 9.00 e) None of these. 10. Determine the ph after a total of ml of M NaOH has been added. a) 8.00 b) 8.85 c) d) e) None of these. 11. Determine the volume of M NaOH required to achieve a ph of a) 83.1 ml b) 78.4 ml c) 72.1 ml d) 54.5 ml e) 48.7 ml 12. You have 1.00 M HC2H3O2 and 1.00 M NaC2H3O2. You wish to use these to make 1.00 L of a buffer solution with a ph of What volumes of each solution should you mix together? a) 643 ml of 1.00 M NaC2H3O2 and 357 ml of 1.00 M HC2H3O2. b) 643 ml of 1.00 M HC2H3O2 and 357 ml of 1.00 M NaC2H3O2. c) 500. ml of 1.00 M NaC2H3O2 and 500. ml of 1.00 M HC2H3O2. d) 556 ml of 1.00 M NaC2H3O2 and 444 ml of 1.00 M HC2H3O2. e) 556 ml of 1.00 M HC2H3O2 and 444 ml of 1.00 M NaC2H3O You have a saturated solution of aqueous NaCl (contains the maximum concentration of NaCl) and add 0.10 M HCl (aq) dropwise. What will happen? The Ksp value for NaCl is equal to 38. a) The NaCl will precipitate out of solution and continue to do so as more 0.10 M HCl is added. b) At first NaCl will precipitate out of solution, but it will stop doing so after awhile. As more 1.0 M HCl is added, the NaCl solid will remain. c) At first nothing will seem to happen, but as more 0.10 M HCl is added, a precipitate will form. d) At first NaCl will precipitate out of solution, and then the NaCl will begin to redissolve. After awhile there will be no solid left. e) No NaCl will ever precipitate out of solution.

4 PracticeHour Exam II Page No Consider the formation of the complex ion Ag(NH3)2 + (aq) when Ag + (aq) and NH3 (aq) react: Ag + (aq) + NH3 (aq) Ag(NH3) + (aq) K1 = 2.1 x 10 3 Ag(NH3) + (aq) + NH3 (aq) Ag(NH3)2 + (aq) K2 = 8.2 x 10 3 Determine the following equilibrium concentrations when 50.0 ml of 2.00 x 10-3 M AgNO3 is reacted with 50.0 ml of 5.00 M NH [Ag(NH3)2 + ] a) 1.00 x 10-3 M b) 2.00 x 10-3 M c) 2.50 M d) 5.00 M e) 1.7 x 10 7 M 15. [NH3] a) 1.00 x 10-3 M b) 2.00 x 10-3 M c) 2.50 M d) 5.00 M e) 2.1 x 10 3 M 16. [Ag + ] a) 9.31 x M b) 4.65 x M c) 2.32 x M d) 5.23 x 10-9 M e) 1.00 x 10-3 M You mix two aqueous ionic solutions together and they do not react. To this you add (dropwise) a third aqueous ionic solution. At first a white solid forms. As you continue to add the third solution, a yellow solid begins to form. Which of the following can you say with confidence? I. The solubility of the white solid is greater than the solubility of the yellow solid. II. The solubility of the yellow solid is greater than the solubility of the white solid. III. The Ksp value for the white solid is greater than the Ksp value for of the yellow solid. IV. The Ksp value for the yellow solid is greater than the Ksp value for of the white solid. a) I and III b) II and IV c) I only d) II only e) I and IV 18. Which of the following statements is true concerning the relative solubility of silver chloride? All samples are at the same temperature. a) AgCl is equally soluble in 1.0 M HNO3 and pure water. b) AgCl is more soluble in 1.0 M AgNO3 than it is in pure water. c) AgCl is less soluble in 1.0 M HNO3 than it is in pure water. d) AgCl is more soluble in 1.0 M HNO3 than it is in pure water. e) AgCl is equally soluble in 1.0 M AgNO3 and pure water.

5 PracticeHour Exam II Page No Indicate which of the graphs below best represents each plot described. Note: the graphs may be used once, more than once, or not at all. a) b) c) d) e) 19. [H + ] (y) vs. [HA]o (initial concentration) (x) for an aqueous weak acid (constant T). B 20. [H + ] (y) vs. [OH - ] (x) for an aqueous solution at constant temperature. D

6 PracticeHour Exam II Page No ml of a solution of a weak acid HA (Ka = x ) in water has a ph = Calculate the amount of water that must be added to reach a ph value of Show all work L

7 PracticeHour Exam II Page No The gravimetric determination of calcium lab in Chemistry 203 had many examples of some interesting chemistry. At the time of the lab we had not yet discussed some of the chemistry involved, so you were left with stoichiometric and error propagation calculations. We can rectify that now! a) To precipitate the calcium oxalate out of solution you added ammonium oxalate, (NH4)2C2O4. Having an acidic solution helped to ensure slow precipitation so you added a great deal of acid; this is one of the reasons we started with ammonium oxalate it is slightly acidic. Determine the ph of M (NH4)2C2O4. Show all work. ph = 6.58

8 PracticeHour Exam II Page No (con t) b) We chose CaC2O4 as a precipitate because of its low Ksp value (2 x 10 9 ). Calcium oxalate is more soluble in acidic solution than in pure water at the same temperature, however, so you raised the ph. Determine the ratio of solubility (M) of CaC2O4 in a solution with a ph of 1.00 solubility (M) of CaC2O4 in pure water Show all work. (To simplify matters, assume the precipitate is CaC2O4 and not CaC2O4 H2O and ignore the basic properties of the oxalate ion in pure water) Ratio = 48

9 PracticeHour Exam II Page No Mix equal volumes of one solution from Group I with one solution from Group II to achieve the results below. Explain how you know which to choose and calculate the ph of each solution. Show all work. Group I: 0.20 M NH4Cl, 0.20 M HCl, 0.20 M C6H5NH3Cl, 0.20 M (C2H5)3NHCl Group II: 0.20 M KOI, 0.20 M NaCN, 0.20 M KOCl, 0.20 M NaNO2 a) The solution with the lowest ph. b) The solution with the highest ph. c) The solution with the ph closest to a) ph = 2.21 (HCl and NaNO2) b) ph = [(C2H5)3NHCl and KOI] c) ph = 7.00 [(C2H5)3NHCl and NaNO2]