Chemistry 1A, Fall 2011 Midterm Exam #2 October 18, 2011 (90 min, closed book) Name:__________________________________ SID:___________________________________ GSI Name:________________ - The test consists of 4 short answer questions and 23 multiple choice questions. - Put your written answers in the boxes provided. Answers outside the boxes will NOT be considered in grading. - Write your name on every page of the exam. Question Page Points Score Multiple Choice 2-8 76 Cabbage juice 2 6 Kitty Litter 3 6 Nicotine 6 4 Caffeine 8 8 Total 100 Useful Equations and Constants: pH = - log[H+] pX = - log X X = 10-pX Kw = 1×10-14 at 25°C PV = nRT ][][logHAApKpHa R = 0.0821 L atm mol−1 K−1 T (K) = T (°C) + 273 Strong acids and bases: HCl LiOH HNO3 NaOH H2SO4 KOH HClO4 HBr HI Acid dissociation constants (Ka) CH3COOH 1.75×10-5 NH4+ 5.70×10-10 HCN 6.2×10-10 CH3NH3+ 2.3×10-11 H2CO3 4.45×10-7 (CH3)3NH+1.58×10-10Name_____________________GSI__________ Page 2 of 9 CABBAGE JUICE INDICATOR Cabbage juice indicator contains two weak acids: anthocyanin, HA, and anthoxanthin, HB. Data on red cabbage juice are given in the tables below. Anthocyanin: HA (aq)  H+ (aq) + A− (aq) red blue Anthoxanthin: HB (aq)  H+ (aq) + B− (aq) colorless yellow Red cabbage pH indicator colors pH 2 4 6 8 10 12 color red red-purple purple blue blue- green green 1. Use the data to estimate Ka for anthocyanin. A) 107 B) 10-2 C) 10-6 [2] D) 10-11 2. The major species in a cabbage juice solution at pH = 2 are: A) HA, A−, HB B) A−, HB C) A−, B− D) HA, HB [4] 3. Suppose you have a purple solution of cabbage juice. What happens to the color if you add water? A) Nothing B) Turns a lighter shade of purple [2] C) Turns more blue [4] D) Turns more red [2] SHORT ANSWER: At pH > 12 the red cabbage juice solution is green. Over time, the solution turns yellow. Present a hypothesis as to why this happens. Above pH 12 the major species present are A− (blue) and B−(yellow) which gives a green solution. Since the solution turns yellow over time the A− must decompose. (2 points) Use this hypothesis to predict what you would observe if you added acid to the yellow solution to neutralize it. If you add acid to a solution of A− and B−, the pH should drop forming more HA and HB, which would turn the solution from yellow to green to blue (and maybe even purple if you add enough acid). But if A− has decomposed HA won’t form so the solution would probably just turn from yellow (B−) to colorless (HB). (4 points)Name_____________________GSI__________ Page 3 of 9 KITTY LITTER Cat urine has a strong, unpleasant odor due to ammonia, NH3. Fortunately, kitty litter not only absorbs moisture, but it also absorbs much of the odor. Many kitty litters are made of clay, which absorbs water and ammonia. A certain brand of kitty litter was tested for its ability to absorb NH3 (g). The kitty litter and NH3 are in a closed 10 L container at 25°C. NH3 (g)  NH3 (clay) # of moles of NH3 absorbed Mass of clay Partial pressure of NH3 2.7 moles 0.1 kg 0.10 atm 4. Use the test results to determine if there is more NH3 in the clay or in the gas phase. Which of the following statements is true? A) Most of the NH3 is absorbed in the clay. [2] B) Most of the NH3 remains as a gas. C) NH3 (g) is in equilibrium with NH3 (clay), so both are present in equal amounts. 5. In the test described above, what happens if the volume of the container is decreased to 5 L with the same total # of moles of NH3 at 25°C? A) The partial pressure of NH3 is 0.20 atm. B) The # of moles of NH3 absorbed in the clay increases. [2] C) There is no change because the equilibrium constant does not depend on pressure. 6. The kitty litter is exposed to other gases. Which gas requires the smallest partial pressure to result in absorption of 1 mole of gas per 0.10 kg of clay? A) O2 B) He C) CO2[2] D) H2S [4]Name_____________________GSI__________ Page 4 of 9 SHORT ANSWER: Vinegar (acetic acid) can also be used to reduce the odor of cat urine. Write the reaction between vinegar (CH3COOH) and ammonia (NH3). CH3COOH + NH3  CH3COO−NH4+ (2 points) Explain why this reaction reduces the odor. The product, ammonium acetate (CH3COO−NH4+) is ionic and will likely not be able to go into the gas phase. It will either be an ionic solid or remain aqueous if there is enough liquid present. (4 points) ORGANIC BASES Nucleic acids, proteins, neurotransmitters, and stimulants are all organic bases that contain nitrogen atoms. Use the data below to answer the questions about organic bases. Name Formula Lewis Structure Kb pKb methylamine CH5N 4.4 × 10−4 3.4 ammonia NH3 2 × 10−5 4.7 methylene methanamine C2H5N 10−7 7 acetonitrile C2H3N 10−24 24 7. Which solution is the most basic? A) 0.10 M CH5N [3] B) 0.10 M NH3 C) 0.10 M C2H5NName_____________________GSI__________ Page 5 of 9 8. What is the pH of a 0.10 M solution of methylamine (CH5N)? A) 2 B) 5 C) 9 D) 12 [4] 9. What is the pKa for the conjugate acid of methylamine? A) 2.3 × 10−11 B) 10.6 [2] C) 4.4 × 10−4 D) 3.4 10. The best explanation for the trends in Kb is A) Changes in hydrogen bonding B) Changes in molar mass C) Changes in the hybridization around the N atom [3] D) Changes in the number of H atoms attached to the C atoms E) Changes in the electron density around the N atom [4] 11. Which species below is the strongest acid? A) NH4+ [3] B) NH3 C) CH5NH+ [1] D) CH5N 12. Which statement is correct about the difference in pKb between methylamine, CH5N, and ammonia, NH3? A) Methylamine is more basic because the CH3 group shifts electron density towards the N atom. [3] B) Methylamine is more basic because the CH3 group shifts electron density away from the N atom. C) Methylamine is more basic because electron density shifts away from the N atom due to resonance structures. 13. Ammonium chloride, NH4Cl, produces acidic solutions. When cobalt ions, Co2+, are added to NH4Cl (aq), the complex cation, Co(NH3)62+ forms. What happens to the pH of the solution? A) The pH decreases because NH3 is removed and more NH4+ dissociates. [3] B) The pH decreases because Cl- is a weak conjugate base. C) The pH increases because Co2+ is a strong acid. D) The pH stays the same because NH4Cl (aq)