Unformatted text preview: Chem 6AL W10 Russak 1 February 2010 Midterm 1 KEY The following questions are all multiple choice. Please fill out the score sheet with your name. You must put your perm number on the score sheet and fill in the bubbles associated with it to receive credit for the exam. There are equations on the back sheet that you may need to calculate your answers. You may use a calculator. There is only 1 correct choice for each question, filling in multiple answers will result in no credit given. 1. What is the weight percent of glucose in a solution made by dissolving 24.6g of glucose in 145.2g of water? a. 1.5% b. 1.7% c. 14.5% d. 16.9% e. None of the above 2. How would you prepare 400g of a 2.5 wt% solution of NaCl? a. Dissolve 400g of NaCl in 2500 mL of water b. Dissolve 10 g of NaCl in 390 mL of water c. Dissolve 10 g of NaCl in 400 mL of water d. Dissolve 2.5 g of NaCl in 397.5 mL of water e. None of the above 3. Convert the following to mol percent: a 25 wt% solution of compound X in compound Y. (X=159 g/mol, Y =98 g/mol) a. 17.0 mol % b. 35.1 mol % c. 38.1 mol % d. 61.9 mol % e. None of the above 4. According to the laws of colligative properties, adding NaCl to water would a. Increase the boiling point of the solution b. Decrease the boiling point of the solution c. Increase the melting point of the solution d. Both B and C e. None of the above 5. Colligative properties are those properties that depend on a. The mass of the particles b. The quantity of particles c. The density of particles d. The refractive index of particles e. All of the above 6. A soon to be bride takes her ring to the jeweler because she suspects her fiancé of cheeping out. The jeweler finds that the refractive index is 2.20. If the speed of light that travels through diamond is 1.24 x 108 m/s, is this a fraud? a. No, she can go ahead and get married b. No, because diamond would have a lower refractive index c. No, because a refractive index of lower that 1 is impossible d. Yes, because a refractive index of greater than 1 is impossible e. Yes, because diamond would have a higher refractive index 7. The refractive index of Cyclohexane is ________ if the speed at which light travels through it is 2.33 E8 m/s. a. 0.700 b. 1.287 c. 1.428 d. 2.184 e. None of the above 8. What can be said about the range of a melting point? a. A broad range is indicative of a pure compound b. The range is not an indicator of purity c. A sharp range is always indicative of a pure compound d. The broader the range, the more impure the compound e. None of the above 9. What does a melting point actually represent? a. The volume required for a solid to go to a liquid b. The energy required to disrupt the crystal lattice of a solid c. The energy required for a liquid to go to a gas d. The energy required for a solid to go to a gas e. None of the above 10. A mixed melting point experiment would be best suited for what kind of scenario? a. When you have two unlabeled bottles and you don’t know their possible identities b. When two compounds have the same or a very similar melting point c. When the sample to be taken is wet d. Determining the heat of mixing of two solids e. None of the above 11. In experiment 1, two students perform the same melting point on the same pure sample of Urea. One student gets 131‐133 °C and the other gets 95‐110 °C. The literature is around 133 °C, so what most likely went wrong with the second student? a. The sample of urea contained contaminants b. The melting point was taken too slowly and it melted before reaching the right temp c. The melting point was taken too fast d. The sample was a eutectic mixture e. None of the above 12. What physical data would you need in order to convert 5 mL of liquid X to moles? a. molecular weight of X b. refractive index of X c. density of X d. a and c e. None of the above 13. The refractive index of Cyclohexane is ________ if the speed at which light travels through it is 2.33 E8 m/s. a. 0.700 b. 1.287 c. 1.428 d. 2.184 e. None of the above 14. What factors influence the boiling point? a. The surface area of the molecule b. The intermolecular forces c. The number of particles d. The concentration of the liquid e. All of the above 15. How should cooling water be attached to a water‐jacketed reflux condenser? a. The water should flow in at the lowest point and out at the highest b. The water should flow in at the highest point and out at the lowest c. The condenser should be filled and then the water turned off so it doesn’t interfere in the experiment d. A and C e. B and C 16. You are planning on performing an experiment that calls for a starting material with at least 75% purity. You find the material on your chemical shelf, but it is a pretty dark liquid (literature says it is supposed to be clear) and there isn’t very much of it. You need as much as possible. Based on this information, what purification method would you perform? a. Perform a recrystallization b. Perform a simple distillation c. Perform a fractional distillation d. Perform a sublimation e. Any of the above would be adequate Use the graph below to answer the following questions. 17. A solution of 70/30 hexane/pentane was distilled once. What is the approximate purity of the distillate according to this graph? a. 99/1 hexane/pentane b. 70/30 pentane/hexane c. 85/15 pentane/hexane d. 90/10 hexane/pentane e. None of the above 18. According to the graph above, in order to purify a sample of 60/40 hexane/pentane one must heat until the temp of the solution is a. 42 °C b. 47 °C c. 56 °C d. 62 °C e. None of the above 19. What is true of vapor pressure? a. The higher the vapor pressure, the higher the boiling point b. Vapor pressure is not a colligative property c. The lower the vapor pressure, the lower the boiling point d. The higher the vapor pressure, the lower the boiling point e. A and B 20. Consider a 33 mol % mixture of dichloromethane (P°=47 kPa) in ether (P°=53 kPa). What is the combined vapor pressure? a. 6 kPa b. 49 kPa c. 51 kPa d. 100 kPa e. None of the above 21. From the question above, calculate the fraction of dichloromethane molecules that reside in the vapor above the solution? a. 20 % b. 25% c. 31% d. 66 % e. None of the above 22. Let’s say you have an organic compound with a pKa of 5. In an organic/aqueous system, you add a base where the pKa of the conjugate acid is 8. If the organic solvent is diethyl ether, then where is the organic compound? a. The top organic layer b. The top aqueous layer c. The bottom organic layer d. The bottom aqueous layer e. None of the above 23. Consider that you have a partition coefficient of 3 for a substance in an ether/water solution. You perform 3 extractions using ether. What is the total amount of material in the organic layer assuming equal volumes of liquids were used? a. 1.5% b. 25% c. 75% d. 98.5% e. None of the above 24. Compound-W has a partition coefficient value (K) of 2.5 in a 2:1 v/v (volume/volume) of
CH2Cl2:H2O. If 1g of compound-W is successively partitioned between equal volumes of CH2Cl2:H2O, how much of compound-W will be in the combined CH2Cl2 layers after the second separation? 3% 20% 80% 97% None of the above 25. A brine wash is used to remove water from organic liquids. It works by: a. Increasing the concentration of NaCl in the water later b. Decreasing the concentration of NaCl in the water layer c. Trapping water in small pockets that they cannot be removed from easily d. Forming hydrates e. None of the above a. b. c. d. e. 26. The stretch of a carbonyl of a ketone is near 1720 cm‐1 and the C‐O stretch of an ether is around 1200 cm‐1. What is true based on this information a. The ketone stretch is lower in energy than the ether b. The ketone stretch is higher in energy than the ether 27. A stretch in the infrared was recorded to be 3.37 E‐20 Joules. What functional group does this correspond to? a. O‐H b. C‐H c. C=O d. C‐O e. None of the above Match the following structures to their IR spectra O O O H OH A
28. D B C D E 29. E 30. B 31. C 32. A Equations: N=c/ʋ E=hc/λ h=6.626 E‐34 J.s c = 3.00 E8 m/s P=P°X P=PA+PB Υ=P/Ptotal K = [A]org / [A]aq [A]aq final / [A]aq initial = [(V2/(V2+V1K)]n pKa = ‐log Ka ...
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- Winter '07
- Solvent, refractive index, Partition coefficient