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 Useful Equations and Constants Write your name on every page of the exam pH log H pX log X Question Page Points Score X 10 pX Kw 1 10 14 at 25 C Multiple Choice 2 8 76 PV nRT Cabbage juice 2 6 Kitty Litter 3 6 Nicotine 5 4 Caffeine 7 8 Total 100 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 10 Name GSI 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 red H aq A aq blue Anthoxanthin HB aq colorless H aq B aq yellow Red cabbage pH indicator colors pH 2 4 6 8 10 12 color red redpurple purple blue bluegreen green 1 Use the data to estimate Ka for anthocyanin A 107 2 B 10 2 D 10 11 The major species in a cabbage juice solution at pH 2 are A HA A HB B A HB 3 C 10 6 C A B D HA HB 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 Page 2 of 8 Name GSI 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 soaks up 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 2 7 moles 4 5 6 Mass of clay 0 1 kg Partial pressure of NH3 0 10 atm 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 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 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 C There is no change because the equilibrium constant does not depend on pressure 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 D H2S 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 Explain why this reaction reduces the odor Page 3 of 8 Name GSI 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 methylamine Lewis Structure Kb pKb 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 8 B 5 C 9 D 12 What is the pKa for the conjugate acid of methylamine A 2 3 10 11 10 C 0 10 M C2H5N What is the pH of a 0 10 M solution of methylamine CH5N A 2 9 B 0 10 M NH3 B 10 6 C 4 10 4 D 3 4 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 D Changes in the number of H atoms attached to the C atoms E Changes in the electron density around the N atom Page 4 of 8 Name GSI 11 Which species below is the strongest acid A NH4 12 13 14 B NH3 C CH5NH D CH5N 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 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 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 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 is a buffer The nicotine molecule is shown below along with pKb for each N atom What is the predominant form of nicotine present in the blood at pH 7 2 A B C D Page 5 of 8 Name GSI 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 pH Volume HCl added 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 17 B 40 C C 50 C Why do farmers spray water on oranges to protect them from freezing on a cold night A When water freezes …