University of Maryland--Baltimore County Chemistry 351L Fall, 1994 Midterm Exam Part I, Multiple Choice--Answer the following questions on the Scantron machine-readable answer sheet provided. 1. Which of the following statements is not true. A) Azobenzene is a suspected carcinogen. B) The greatest danger associated with the use of diethyl ether is that it can be ignited by a hot surface. C) A closed system should never be heated. D) Dichloromethane is highly flammable. E) All chemicals should be treated as if they were toxic. 2. What is the correct way to order the following eluants in order of increasing polarity? A) acetone H3O+ + CN- which pair of substances both are bases: A) H2O and CN- B) H3O+ and H2O C) HCN and H3O+ D) HCN and CN- E) H3O+ and CN- Answer Key: 1. D 4. D 7. D 10. E 2. B 5. B 8. B 11. A 3. B 6. B 9. B Part II, Free Response--Answer the following questions on the free- response answer sheet provided. 12. The student next to you at the bench has taken off her/his safety glasses, having finished the experiment for the day. Suddenly your apparatus for dehydration of an alcohol bumps from local superheating; blows off the distillation head; and splashes sulfuric acid solution into the eyes of the student without glasses. What emergency and first aid procedures do you employ? Guide the student immediately to the eye-wash fountain and direct him to flush for 15 min. 13. You find it necessary to separate a mixture of naphthalene and 2- naphthol by column chromatography. The compounds are dissolved in petroleum ether, introduced onto an alumina column and then eluted with petroleum ether. a. Which compound is eluted first and why? Naphthalene is eluted first because it is less polar than 2-naphthol and is not adsorbed as strongly on the polar stationary phase. b. As the chromatography proceeds the eluate is periodically captured and analyzed by thin-layer chromatography on silica gel using hexane as the mobile phase. In the beginning only one spot is observed at Rf 0.6. After the first compound is eluted another spot representing the other compound in the mixture is observed on the tlc plate using the same conditions. Is the Rf greater or less than 0.6 and why? Rf is less that 0.6. Second spot represents 2-naphthol which is more strongly retained by the TLC plate thus not migrating as far. 14. Examine the accompanying boiling point-composition diagram for mixtures of toluene and benzene. An investigator distilled 50 mL of a mixture that was 75 mole percent toluene. a. What was the mole percent of benzene in the mixture? 25 mole percent The investigator collected about 5 mL of distillate from a simple distillation. She observed a boiling point which did not change appreciably while the 5 mL was being collected. b. What was the observed boiling point? 104 C The investigator then removed the 45 mL of mixture remaining in the distilling flask; transferred the 5 mL of first distillate to the distilling flask: and distilled the 5 mL by simple distillatioin. c. What was the composition of the first few drops of distillate from this second distillation? 32 mole percent toluene 15. Suppose the distribution coefficient for caffeine between ether and water is 5 (K = caffeine concentration in ether/caffeine concentration in water). a. What fraction of caffeine in 50 mL water is extracted into 10 mL ether? x = fraction extracted into ether x/10 K = 5 = _____ (1-x)/50 5 = 5x/(1-x) x = 0.5 b. What fraction of caffeine in 50 mL water is extracted into two 5 mL portions of ether? x/5 K = 5 = _____ (1-x)/50 5 = 10x/(1-x) x = 0.333 extracted into ether which means 0.667 remains in aqueous. After two 5 mL extractions 0.667 x 0.667 = 0.4444 remains in aqueous which means 0.5556 is extracted into ether 16. Phenol is soluble in aqueous sodium hydroxide but not in aqueous sodium bicarbonate, NaHCO3, solution, whereas benzoic acid is soluble in both solutions. Using a flow chart and chemical equations for all acid-base reactions taking place, show how you would separate a mixture of phenol, benzoic acid and anthracene and recover the neutral solids. For convenience in writing the equations use PhOH as an abbreviation for phenol and PhCO2H as an abbreviation for benzoic acid (where Ph stands for phenyl). PhOH & PhCO2H & anthracene dissolve in CH2Cl2; extract with NaHCO3 | CH2Cl2 | NaHCO3 + PhCO2H ---> H20 + CO2 + PhCO2- __________|________________________ | | | | PhOH & anthracene PhCO2- extract with NaOH neutralize with H+ | PhCO2- + H+ ---> PhCO2H | | CH2Cl2 | NaOH + PhOH ---> H20 + PhO- | ______|_______________ | | | filter precipitate | | anthracene PhO- recovered neutralize with H+ by distillation PhO- + H+ ---> PhOH | | filter precipitate