TOTAL /103. March 26, 2003

Transcription

1 March 26, 2003 Name Chemistry Prof. Mines Exam 2-Practice (103 points) [Questions from exams in Spring 2003] [NOTE: I have stressed certain things this semester that I may not have stressed in past semesters. As always, you should use the problem sets as your guide to what is fair game on any of my exams.] This exam consists of 10 questions (many with multiple parts) on six pages. 1) Please briefly scan through the whole exam to make sure that you have all the pages (and so that you can make the best use of your time). 2) Do not spend too much time on any one problem in the first half of the test period. 3) Do questions that you feel you can do quickly first and come back later to questions you are unsure about. 4) Take notice of the point value of the problems to help you judge your time and efforts (see chart below!). 5) Make sure to report all calculated values to the proper precision and show your work (unless you are told otherwise). 6) Remember, don t panic if you don t immediately see the answer; allow yourself time to analyze each problem! Good luck! Problem Type Received/Possible 1 Mult. Choice /16 2 T/F /10 3 /7 4 Free /21 5 Response /17 6 (77 pts) /8 7 /8 8 /6 9 /6 10 /4 TOTAL /103

2 Page 1 1. Circle the correct answer (16 pts total) (2 points each) (a) A system at equilibrium will shift from its equilibrium position if: i) a catalyst is added ii) the concentration of reactants is increased iii) T is lowered iv) all of the above v) (ii) and (iii) only (b) If the ph at the equivalence point of a titration (carried out at 25 C using a strong titrant) was 5.5, then the species being titrated was a i) weak base. ii) strong base iii) weak acid iv) strong acid v) could be (i) or (ii) vi) could be (iii) or (iv) (c) Given the equation AB(s) A + (aq) + B - (aq), if the beaker to the right represents a system in which Q < K, which picture best represents what the system might look like after some time passes? use for (c) A + (aq) B - (aq) solid salt AB (i) (ii) (iii) (iv) (d) One expects a 0.10 M solution of HClO 4 at 25 C to have a ph of i) less than 1 ii) 1 iii) greater than 1 but less than 7 iv) 7 v) greater than 7 because HClO 4 is a i) strong acid ii) weak acid iii) negligible base or acid iv) weak base v) strong base (e) On a cool night, a reptile s blood might be 20. C, at which temperature K w = 6.8 x If the ph of its blood is 7.0, the blood is: (i) basic (ii) acidic (iii) neutral (iv) need more information (f) In the titration of 50.0 ml of M HOCN with 50.0 ml of M NaOH, the properties of the solution at the equivalence point will be identical to the properties of: (i) a M NaOH solution. (ii) a M NaOCN solution. (iii) a M HOCN solution. (iv) a M NaOCN solution. (v) a HOCN / NaOCN buffer (g) A mixture of which pair of 0.1 M aqueous solutions would constitute a buffer? (i) NaOH and NH 3 (ii) NH 3 and NH 4 Cl (iii) HBr and KBr (iv) HNO 3 and NH 4 NO 3

3 Page 2 (h) If a some strong acid is added to a buffer, (i) the moles of base component go up and the moles of acid component go down (ii) the concentrations of the acid and base components of the buffer do not change (iii) some of the base component is converted into the acid component (iv) the ph remains exactly the same 2. Write T (True) or F (False) for the following statements. If the statement is false correct it by changing, dropping, or adding a few words (10 points in total; 2 points each) a) If a 0.10 M solution of HC 2 H 3 O 2 is less acidic than a 0.10 M solution of HCN, then CN - is a weaker base than C 2 H 3 O 2 -. b) In a 0.20 M solution of weak acid, HA, [HA] is much greater than [A - ], and [A - ] is approximately equal to [H 3 O + ]. c) The greater the pk a value is, the stronger the acid. d) When a system comes to equilibrium, no more reaction occurs in the forward or reverse directions. e) If chemical equation 3 is the sum of equations 1 and 2, then the equilibrium constant for equation 3 is the sum of the equilibrium constants for equations 1 and 2. Free Response Questions (72 points total) 3. (7 pts) (a) (2 pts) Identify the equivalence point on the following titration curve of ml of a monoprotic acid (or base) with a strong base (or acid). 14 (b) (2 pts) Estimate the pk a of the substance being titrated. 12 (c) (3 pts) If the titrant was M (either NaOH or HCl), what was the initial concentration of the substance being titrated? ph (ml) Volume of titrant added

4 Page 3 4. (21 pts) Consider the following equation: CCl 4 (g) C(s) + 2 Cl 2 (g) for which K c = at 427 C. (a) (4 pts) Write the equilibrium constant expression. (b) (5 pts) If some CCl 4 is put into a sealed container at 427 C., do you expect that the majority of it will be converted into products before equilibrium is established? Explain briefly. (c) (4 pts) If a system containing CCl 4 (g), C(s), and Cl 2 (g) were at equilibrium and the volume of the container were doubled, would the system shift, and if so, in which direction would net reaction occur? Explain briefly. (d) (6 pts) If 3.2 moles of CCl 4 is put into a 1.0 L container at 427 C, set up a mathematical equation that would allow you to determine the concentrations at equilibrium. Solve this in the NEXT part. (e) (2 pts) Use your equation in (d) to solve for the concentrations of Cl 2 and CCl 4 at equilibrium. Comment on whether or not your answers are consistent with your prediction in (b). (NOTE: This part is only worth 2 points, so you might consider doing this after you have finished the rest of the exam.)

5 Page 4 5. (17 pts) NOTE: The K a for HClO is 3.5 x 10-8 at 25 C (a) (2 pts) What is the conjugate base of HClO? (no explanation needed) (b) (3 pts) Write the chemical equation corresponding to base ionization of the conjugate base of HClO (your answer to part (a)), and then write the corresponding equilibrium constant expression (i.e., the expression for K b ). (b) (6 pts) Calculate the ph of a solution of M NaClO at 25 C. (c) (6 pts) Calculate the ph of the solution formed when ml of M HClO is mixed with ml of M NaClO at 25 C.

6 Page 5 6. (8 pts) Consider: H 2 PO HCN (a) (4 pts) If HCN is acting as the Bronsted-Lowry acid, write the products of the reaction and identify each one as either a conjugate acid or a conjugate base. (b) (4 pts) If H 2 PO 4 - is a weaker base than the conjugate base you wrote in (a), will the reaction depicted above be product-favored or reactant-favored? Explain briefly. 7. (8 pts) Given: K sp [Al(OH) 3 ] = 1.9 x at 298 K. (a) (6 pts) What is the molar solubility of Al(OH) 3 in water at 298 K?. (b) (2 pts) What is the molar solubility of Al(OH) 3 at 298 K in a solution of 1.0 M NaOH? 8. (6 pts) Some drinking water at 25 C was found to have a ph of 8.2. What was the concentration of OH - in the water?

7 Page 6 9. (6 pts) K c = 4.1 at 300 C for the following equation: PCl 3 (g) + Cl 2 (g) PCl 5 (g) If a 2.0 L box at 300 C were filled with 2.1 moles of PCl 3, 1.3 moles of Cl 2, and 7.2 moles of PCl 5, would any net reaction occur in the forward or reverse direction? Justify your answer. 10. (4 pts) State whether an aqueous solution of each compound would be acidic or basic and briefly note your reasoning. (a) KSCN (b) NH 4 Cl