Chemistry 201: General Chemistry II - Lecture

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1 Name Date Chemistry 201: General Chemistry II - Lecture Short-Answer Exam #2, 60 Points Total Form: A Read all directions carefully. Answers not conforming to the directions will be marked as incorrect! Section 1: Matching (10 points) Match the following concepts with the appropriate definitions or examples listed below. Make selections such that each term has its best possible match. Indicate your choices by marking your Scantron sheet (1-10) with the appropriate two-letter code. endothermic strong acid law of mass action K w Q hydronium buffer ph neutral salt solutions conjugate pairs AC. heat is a reactant CD. could be acidic, neutral, or basic DE. created when conjugate acid/base pairs are in the same solution AB. relationship between balanced equation and K eq CE. ionizes completely AE. two compounds which differ by only a proton BC. H 3 O + or H + for short BD. when [H 3 O + ] = [OH ] AD. reaction quotient BE. equals at 25 C 1

2 Section 2: Review Topics Multiple Choice (5 points Extra Credit) Choose the term or phrase that best completes the sentence or answers the question. Indicate your selection by marking the appropriate letter on your Scantron sheet. 11. The S 2 anion is formed when is placed in water. S 8 SO 2 Na 2 SO 4 K 2 S 12. What volume of M Cr 2 O 2 7(aq) is required to oxidize 60.0 ml of M Fe2+ (aq) according to the reaction Cr 2 O 2 7(aq) + 6Fe 2+ (aq) + 14H + (aq) 2Cr 3+ (aq) + 6Fe 3+ (aq) + 7H 2 O (l)? 25.0 ml 42.0 ml 58.4 ml 175 ml 13. A student wants to prepare 25 0 ml of 0.10 M NaCl (MW = 58.4 g/mole) solution. Which procedure is most appropriate? Add 5.8 g of NaCl to 25 0 ml of H 2 O. Add 1.5 g of NaCl to 25 0 ml of H 2 O. Dissolve 5.8 g of NaCl in 50 ml of H 2 O and dilute to 25 0 ml. Dissolve 1.5 g of NaCl in 50 ml of H 2 O and dilute to 25 0 ml. 14. A 25.0 ml solution of 2.0 M calcium hydroxide contains moles of hydroxide

3 15. Which of the following is a strong electrolyte? CH 3 COOH KNO 3 PbI 2 BaSO 4 Section 3: Multiple Choice (40 points) Choose the term or phrase that best completes the sentence or answers the question. Indicate your selection by marking the appropriate letter on your Scantron sheet. 16. If an equilibrium equation is multiplied by a constant factor, then its equilibrium constant is. inverted divided by the same factor multiplied by the same factor raised to the same factor 17. When 0.10 M solutions of HF, HCl, KF, and KCl are arranged in order of increasing ph which order is correct? HF, HCl, KF, KCl HCl, HF, KF, KCl HCl, HF, KCl, KF HF, HCl, KCl, KF 18. A 60.0 g sample of CaCO 3 is heated to 950 K in a 1.00 L evacuated container, where it reacts according to the equation: CaCO 3(s) CaO (s) + CO 2(g). After equilibrium is attained, the pressure of CO 2(g) is 30.0 mm Hg. When the experiment is repeated using g CaCO 3, what is the equilibrium pressure P? 15.0 mm Hg P < 30.0 mm Hg P = 30.0 mm Hg 30.0 mm Hg < P < 60.0 mm Hg P = 60.0 mm Hg 3

4 19. When dissolved in water to make solutions of the same concentration, which of the following would form the most basic solution? Na 2 CO 3 NaClO 2 NaNO 2 NaCN 20. Which of the following is true about the symbol? It indicates a partial reaction takes place. It indicates that products can reform reactants. It implies that a complete stoichiometric reaction takes place. All of these could be true; it depends on the specific reaction. 21. A M solution of a weak acid has a ph of What is the value of the K a for this acid? Which of the following is an oxyacid? H 2 O Fe 2 O 3 Cu(OH) 2 H 2 CO Binary acid strength increases as. the number of O s in the formula increases you go down and left on the periodic table you get closer to F as you go down and right on the periodic table 4

5 24. The ph at the equivalence point when 25 ml of a 0.20 M HClO 4 solution is titrated with 0.10 M LiOH is ml The K b of ClO 4 is needed to solve this problem. 25. In a titration, an indicator is used to visualize the which is not necessarily the same as the. endpoint/equivalence point equivalence point/endpoint the neutralization point/endpoint pk a /[H 3 O + ] 26. Which of the following is a conjugate of H 2 PO 2 4? PO 3 4 H 2 PO 5 H 3 PO 4 H 4 PO When dissolved in water, forms an acidic solution. NaCl CH 3 COONa FeCl 3 NH Which of the following pairs of species, when in the same solution, do NOT create a buffer? HCN/CN NH 3 /NH + 4 H 3 O + /OH All of these create a buffer solution. 5

6 29. IBr (g) is in equilibrium with I 2(g) and Br 2(g) at 150 C: 2IBr (g) I 2(g) + Br 2(g), K = Initially, a closed vessel at 150 C has a partial pressure of IBr of atm and partial pressures of I 2 and Br 2 each of atm. What is the partial pressure of IBr once the system reaches equilibrium? 1.01 atm 1.09 atm 1.56 atm 1.82 atm 30. Which of the following has the strongest conjugate acid? Cl NH 3 HNO 3 F 31. The Henderson-Hasselbalch equation would predict an incorrect ph for a buffer made with which of the following acids and their respective conjugates? HSCN (K a = ) HN 3 (K a = ) HOCl (K a = ) C 6 H 5 OH (K a = ) 32. The K a of hydrocyanic acid, HCN, is What is the ph of M HCN (aq)? between 3.5 and 4.5 between 5.0 and 5.5 between 9.0 and 9.5 between 10.5 and

7 33. For which reaction at equilibrium will a decrease in volume at constant temperature cause a decrease in the amount of product? N 2(g) + 3H 2(g) 2NH 3(g) HCl (g) + H 2 O (l) H 3 O + (aq) + Cl (aq) Fe 3 O 4(s) + 4H 2(g) 3Fe (s) + 4H 2 O (g) CaCO 3(s) CaO (s) + CO 2(g) 34. Which of the following is amphoteric? HNO 3 H 2 BO 3 CH 3 COO S Which group on the periodic table commonly forms compounds that are Lewis acids? Group 13 Group 15 Group 17 Group 18 7

8 Section 4: Short Answer (5 points each) 36. What effect does adding or removing heat have on a reaction that is at equilibrium? 37. Explain why conjugate bases of strong acids are ph neutral. Using a specific example may help clarify your answer. 8

9 Answer Key for Exam A Section 1: Matching (10 points) (1) endothermic (5) strong acid (4) law of mass action (10) K w (9) Q (7) hydronium (3) buffer (8) ph neutral (2) salt solutions (6) conjugate pairs AC. heat is a reactant CD. could be acidic, neutral, or basic DE. created when conjugate acid/base pairs are in the same solution AB. relationship between balanced equation and K eq CE. ionizes completely AE. two compounds which differ by only a proton BC. H 3 O + or H + for short BD. when [H 3 O + ] = [OH ] AD. reaction quotient BE. equals at 25 C 1

10 Section 2: Review Topics Multiple Choice (5 points Extra Credit) Section 3: Multiple Choice (40 points)

11 Section 4: Short Answer (5 points each) 36. Answer: Adding or removing heat (changing the temperature) of a reaction at equilibrium shifts the reaction right or left depending on whether the reaction is endothermic or exothermic. Specifically, changing the temperature changes the value of the equilibrium constant. 37. Answer: Strong acids such as HCl dissociate completely in water, which can be represented by the equation HCl (aq) H + (aq) + Cl (aq). In this example Cl is the conjugate base of HCl. The one-way arrow indicates that this reaction is not reversible, i.e. the reaction Cl (aq) + H+ (aq) HCl (aq) does not happen. If this reverse reaction doesn t happen, it means that Cl cannot pick up a proton and therefore cannot act as a base. Thus, it has no acid/base character and is ph neutral. 3