PracticeMidterm3C

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Chemistry 1A, Section 4 Practice Exam #3C (90 min, closed book) Name:__________________________________ SID:___________________________________ GSI Name:________________ • 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. Useful Equations and Constants: pH = - log[H + ] pX = - log X X = 10 -pX Kw = 1 × 10 -14 at 25 ° C PV = nRT R = 0.0821 L atm mol − 1 K − 1 T (K) = T ( ° C) + 273 Strong acids and bases: HCl LiOH HNO 3 NaOH H 2 SO 4 KOH HClO 4 HBr HI Acid dissociation constants (Ka) CH 3 COOH 1.75 × 10 -5 NH 4 + 5.70 × 10 -10 HCN 6.2 × 10 -10 CH 3 NH 3 + 2.3 × 10 -11 H 2 CO 3 4.45 × 10 -7 (CH 3 ) 3 NH + 1.58 × 10 -10 97

Name_____________________GSI__________ Page 2 of 8 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) D H + (aq) + A − (aq) red blue Anthoxanthin: HB (aq) D 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 K a for anthocyanin. A) 10 7 B) 10 -2 C) 10 -6 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 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 C) Turns more blue D) Turns more red 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. Use this hypothesis to predict what you would you observe if you added acid to the yellow solution to neutralize it. 98

Name_____________________GSI__________ Page 3 of 8 KITTY LITTER Cat urine has a strong, unpleasant odor due to ammonia, NH 3 . Fortunately, kitty litter not only absorbs moisture, but it also absorbs much of the odor. Many kitty litters are made of clay, which soaks up water and ammonia. A certain brand of kitty litter was tested for its ability to absorb NH 3 (g). The kitty litter and NH 3 are in a closed 10 L container at 25°C. NH 3 (g) D NH 3 (clay) # of moles of NH 3 absorbed Mass of clay Partial pressure of NH 3 2.7 moles 0.1 kg 0.10 atm 4. Use the test results to determine if there is more NH 3 in the clay or in the gas phase. Which of the following statements is true? A) Most of the NH 3 is absorbed in the clay. B) Most of the NH 3 remains as a gas. C) NH 3 (g) is in equilibrium with NH 3 (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 NH 3 at 25°C? A) The partial pressure of NH 3 is 0.20 atm. B) The # of moles of NH 3 absorbed in the clay increases. 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) O 2 B) He C) CO 2 D) H 2 S SHORT ANSWER: Vinegar (acetic acid) can also be used to reduce the odor of cat urine. Write the reaction between vinegar (CH 3 COOH) and ammonia (NH 3 ). Explain why this reaction reduces the odor. 99

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Name_____________________GSI__________ Page 4 of 8 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 K b pK b methylamine CH 5 N 4.4 × 10 − 4 3.4 ammonia NH 3 2 × 10 − 5 4.7 methylene methanamine C 2 H 5 N 10 − 7 7 acetonitrile C 2 H 3 N 10 − 24 24 7. Which solution is the most basic? A) 0.10 M CH 5 N B) 0.10 M NH 3 C) 0.10 M C 2 H 5 N 8. What is the pH of a 0.10 M solution of methylamine (CH 5 N)? A) 2 B) 5 C) 9 D) 12 9. What is the pK a for the conjugate acid of methylamine? A) 2.3 × 10 − 11 B) 10.6 C) 4 × 10 − 4 D) 3.4 10. The best explanation for the trends in K b is A) Changes in hydrogen bonding B) Changes in molar mass C) Changes in the hybridization around the N atom D) Changes in the number of H atoms attached to the C atoms E) Changes in the electron density around the N atom 100

Name_____________________GSI__________ Page 5 of 8 11. Which species below is the strongest acid? A) NH 4 + B) NH 3 C) CH 5 NH + D) CH 5 N 12. Which statement is correct about the difference in pK b between methylamine, CH 5 N, and ammonia, NH 3 ? A) Methylamine is more basic because the CH 3 group shifts electron density towards the N atom. B) Methylamine is more basic because the CH 3 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, NH 4 Cl, produces acidic solutions. When cobalt ions, Co 2+ , are added to NH 4 Cl (aq), the complex cation, Co(NH 3 ) 6 2+ forms. What happens to the pH of the solution? A) The pH decreases because NH 3 is removed and more NH 4 + dissociates. B) The pH decreases because Cl- is a weak conjugate base. C) The pH increases because Co 2+ is a strong acid. D) The pH stays the same because NH 4 Cl (aq) is a buffer. 14. The nicotine molecule is shown below along with pK b for each N atom. What is the predominant form of nicotine present in the blood at pH = 7.2? A) B) C) D) 101

Name_____________________GSI__________ Page 6 of 8 15. A sample of nicotine is titrated with 0.10 M HCl solution. Which graph below best represents the titration curve? (Note. The dotted line corresponds to pH 7.) A) B) C) D) SHORT ANSWER: Nicotine is highly toxic. About 60 mg can be a lethal dosage for an adult human. One cigar contains about 120 mg of nicotine. Nicotine in the gas phase can enter the bloodstream. Why doesn’t smoking a cigar kill the smoker instantly? Describe a partition equilibrium in your answer. HEAT AND TEMPERATURE 16. You mix 300 mL of water at 60˚C with 100 mL of water at 20˚C. What is the final temperature of the mixture? A) 30°C B) 40°C C) 50°C 17. Why do farmers spray water on oranges to protect them from freezing on a cold night? A) When water freezes, heat is transferred to the oranges. B) The temperature of the oranges will stay at 0°C until all the water freezes. C) Solid water provides an insulating layer because ice has a high heat capacity D) All of the above pH Volume HCl added 102

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Name_____________________GSI__________ Page 7 of 8 EXTRACTION OF CAFFEINE When tea leaves are soaked in boiling water, several compounds dissolve in the water. Three compounds in high concentration are tannic acid (HT), glucose (C 6 H 12 O 6 ) and caffeine (C 8 H 10 N 4 O 2 ). Use the data below to answer questions about a tea solution. tannic acid, HT glucose, C 6 H 12 O 6 caffeine, C 8 H 10 N 4 O 2 [Note: Tannic acid is abbreviated as HT.] K a (tannic acid) ~ 10 − 6 K a (glucose) ~ 10 − 12 K b (caffeine) = 4.2 × 10 − 4 18. A solution of caffeine is prepared at 80°C by dissolving 10 g caffeine in 100 mL of water. The solution is cooled to 25°C. What is the concentration of the caffeine in that solution? A) 180 mg/mL B) 100 mg/mL C) 78 mg/mL D) 22 mg/mL 19. What is the pH of a 10 − 10 M aqueous solution of tannic acid? A) 7 B) 8 C) 9 D) 10 20. A 1.0 L solution is made by mixing 0.010 mole of tannic acid, 0.0005 mole of caffeine and 0.010 mole of glucose with water. Which ion forms when you add NaOH to the solution? A) T − B) C 6 H 11 O 6 − C) C 8 H 11 N 4 O 2 + Solubility caffeine in water = 22 mg/ml at 25°C; 180 mg/mL at 80°C Solubility of caffeine in CH 2 Cl 2 = 140 mg/ml at 25°C Solubility of glucose in CH 2 Cl 2 = insoluble 103

Name_____________________GSI__________ Page 8 of 8 21. You add 0.005 mole of NaOH to the solution above. The pH of the solution is A) pH ~ 11 because caffeine is the strongest base B) pH ~ 6 because there is a HT/T − buffer C) pH ~ 3 because tannic acid is the strongest acid D) pH ~ 2 because glucose is present in the highest concentration 22. When CaCl 2 is added to a solution of tannic acid, CaT 2 precipitates. What happens to the pH? A) increases B) decreases C) stays the same 23. The K sp for CaT 2 (s) is 2 × 10 − 12 . How much (M) CaT 2 dissolves in water? A) 8 × 10 − 5 B) 1.4 × 10 − 6 C) 2 × 10 − 12 D) 1.6 × 10 − 4 SHORT ANSWER: Calcium hydroxide, Ca(OH) 2 , is added to a tea solution containing tannic acid, glucose, and caffeine. A precipitate forms and the pH increases to 8. Write a balanced chemical equation(s) for the reaction. Next, methylene chloride, CH 2 Cl 2 , is added. Two liquid layers are observed at 25 ° C because CH 2 Cl 2 is not miscible with water. Where are most of the caffeine molecules? Explain your thinking . 104