MCGILL UNIVERSITY FACULTY OF SCIENCE MIDTERM EXAMINATION CHEM 120 MONDAY MARCH 16, :30PM 8:30PM VERSION NUMBER: 1

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1 MCGILL UNIVERSITY FACULTY OF SCIENCE MIDTERM EXAMINATION CHEM 120 MONDAY MARCH 16, :30PM 8:30PM VERSION NUMBER: 1 Instructions: BEFORE YOU BEGIN: Enter your student number and name on the computer scorecard provided, by filling in the appropriate circles. Check that the correct version number filled in. 1) There are 30 multiple choice questions to be answered on the COMPUTER GRADING sheet supplied. Choose the best response to each question. For numerical answers, assume that agreement to the first 2 significant figures is sufficient. Each question is worth 1 mark, and incorrect answers worth 0. There are no part marks available, and ONLY the computer scorecard is to be turned in for grading. You may keep this question sheet, which will allow you to check your results with the answer key posted to the class website after the exam. 2) You may use a simple scientific calculator to assist you, as long as it does NOT have text storage capability. You may also use a language translation dictionary to assist you. NO other materials are permitted (i.e.: this is a closed book exam). Sufficient data are supplied to complete all questions, and scrap paper is appended at the back for any rough calculations. 3) You have 2 hours to complete this exam. Good luck, and work efficiently!

2 UNITS Conventional S.I. Volume ml or cm 3 = cm 3 or 10-3 dm 3 Liter (L) = dm 3 Pressure atm = 760 torr = Pa torr = Pa Temperature C 0 C = K PV L-atm = dm 3 Pa =101.3 dm 3 kpa =101.3 J Energy 1 cal = J (J= kg m 2 /s 2 ) 1 erg = 1 g cm 2 /s 2 = 10-7 J Current ampere (A) = 1 C/s CONSTANTS Avogadro s number N x mol -1 Boltzmann s constant k x J K -1 Faraday s constant F C mol -1 Values of R for various unit combinations cal mol -1 K L atm mol -1 K cm 3 atm mol -1 K J mol -1 K erg mol -1 K dm 3 kpa mol -1 K cm 3 kpa mol -1 K -1 Useful equations: P ln P1 ΔH = R 1 T1 T 2 vap 1 2 PV = nrt m = MPV RT P 1V 1 = n T 1 1 PV n T 2 MP d = P = hdg RT π = imrt C=kP gas P A =χ A P A

3 ΔG (non-standard conditions): ΔG = ΔG + RT ln Q Van t Hoff Equation: o K2 ΔH 1 1 ln = K1 R T2 T1 Standard states for various elements under STP conditions: Hydrogen: H 2 (g) Carbon: C(s, graphite) Nitrogen: N 2 (g) Oxygen: O 2 (g) Copper: Cu(s) Sulphur: S(s) The Periodic Table of the Elements

4 g of an unknown (non-electrolyte) compound is dissolved in g benzene. The resulting solution freezes at 4.97 C. Pure benzene freezes at 5.48 C and has a freezing point depression constant of Kf = 4.90 C/m. What is the molecular weight of the compound? A. 80 g/mol B. 120 g/mol C. 240 g/mol D. 10 g/mol E. 60 g/mol 2. Calculate the temperature at which Keq for a reaction is if ΔH = kj/mol and ΔS = -246 J/mol K. A K B. 307 K C K D. cannot be determined without ΔG E. 274 K 3. The equilibrium constant for the reaction: is 7.2 x 10-4 at 298 K and 1 atm. When [HNO2(aq)] = 1.0 M and [NO2 - (aq)] = [H3O+(aq)] = 1.0 x 10-5 M, calculate ΔG. A kj/mol B kj/mol C kj/mol D kj/mol E kj/mol 4. At the triple point of substance W, the enthalpy of vaporization is 19.4 kj/mol and the enthalpy of sublimation is 28.9 kj/mol. What is the heat of fusion under these conditions? A. 1.5 kj mol-1 B kj mol-1 C kj mol-1 D kj mol-1 E. 9.5 kj mol-1 Page 1

5 5. If ΔG is positive for a certain reaction at a given T and P, then: A. the reaction is spontaneous at high temperatures. B. one would need to know the Kelvin temperature to determine spontaneity. C. the system is in equilibrium. D. the reaction is endothermic E. the reverse of the reaction is spontaneous. 6. Which compound is most likely to be soluble in both water and benzene? A. iodine (I2) B. sodium chloride (NaCl) C. hexane (C6H14) D. isopropanol (C3H7OH) E. toluene (C7H8) 7. For the reaction: Kc = 92.0 When equilibrium concentrations of HI and I2 are [HI] = M and [I2] = M, the equilibrium concentration of [H2] is: A M B M C M D M E M 8. Considering that the following reaction is exothermic, choose the correct statement: A. The reaction is not spontaneous at any temperature. B. The reaction will be spontaneous at all temperatures. C. The reaction will be spontaneous only at low temperatures. D. The reaction will be spontaneous only at high temperature. E. Both forward and reverse reactions are spontaneous. Page 2

6 9. Which of the following is a true statement about chemical equilibria in general? A. At equilibrium, the total concentration of products equals the total concentration of reactants, that is [products] = [reactants]. B. There is only one set of equilibrium concentrations that equals the K c value. C. At equilibrium, the rate of the forward reaction is equal to as the rate of the reverse reaction. D. At equilibrium, the rate constant of the forward reaction is equal to the rate constant for the reverse reaction. E. Equilibrium is the result of the cessation of all chemical change. 10. Surface tension is thought to be due to: A. surface area trying to increase B. solute concentrated on the surface C. air absorbed on the surface D. favourable adhesive interactions E. surface molecules having higher energy than bulk molecules 11. Given the following: Find the value of the equilibrium constant for the following equilibrium reaction: A B C D E Consider the following reaction: At equilibrium at a certain temperature, [H2O(g)] = 0.12 M, and [CO(g)] = [H2(g)] = 1.2 M. If suddenly these concentrations are increased by 0.50 M, which of the following is true? A. Since Kc does not change, nothing happens. B. All the additional H2O(g) is consumed C. Kc = 4.66 D. more products are formed E. more H2O(g) will be formed Page 3

7 13. Assume that seawater is an aqueous solution of NaCl with a concentration of 35 g/l. If the van t Hoff factor for NaCl is i=1.9, what is the osmotic pressure of seawater, if it is separated from pure water by a semi-permeable membrane at 20 C? A. 1.0 atm B atm C atm D. 1.9 atm E atm 14. What is the molarity of a solution that is 26.0% by mass phosphoric acid (H 3 PO 4 ) and that has a density of g/ml? A M B M C M D M E. 300 M 15. The vapor pressures of pure heptane and pure octane at 20 C are mmhg and mmhg respectively. A solution contains mol fraction heptane and mol fraction octane. What is the composition of the vapor in equilibrium with this solution at 20 C? A. 16.5% heptane, 83.5% octane B. 31.0% heptane, 69.0% octane C. 69.0% heptane, 31.0% octane D. 40.0% heptane, 60.0 % octane E. 83.5% heptane, 16.5% octane Page 4

8 16. Based on the boiling-point diagram for a mixture of two substances A & B, shown below, which of the following statements is false? A. A gaseous mixture of 50% A and 50% B will condense at 75 C. B. The vapor above a boiling solution of 75% A contains 25% A. C. The boiling point of pure substance A is 70 C. D. The vapor above a boiling solution of 25% A contains 75% A. E. The boiling point of pure substance B is 90 C. 17. For the reaction: at 298 K, Keq = and ΔS = J/mol K. What is Keq at 200 K if ΔH = 92.6 kj/mol? A B C D E For the reaction: Kp = at 500 K. If a fixed volume is filled with initial concentrations of these gases at 227 C such that [Cl2] = 0.5 M, [H2O] = 0.40 M, [HCl] = 0.5 M, and [O2] = M, in which direction will the reaction proceed? A. The value of Kp at 25 C must be specified to answer this question. B. The reaction is already at equilibrium. C. The reaction volume must be specified to answer this question. D. The reaction proceeds to the left. E. The reaction proceeds to the right. Page 5

9 19. Given the data below, determine the normal boiling point of diethyl ether (CH3CH2OCH2CH3): P1 = 532 mm Hg, T1 = 25.0 C, ΔHvap = 27.2 kj/mol A. 16 C B. 35 C C. 248 C D. 72 C E. 124 C 20. The solubilities of potassium perchlorate, KClO4, in water at 0 C, 20 C, and 80 C are as follows: T ( C) Solubility (g KClO4/100 g H2O) If a solution containing 20 g KClO4 in 250g of water at 80 C is cooled to 20 C, what percentage of the total KClO4 will recrystallize? A. 100 % B. 22 % C. 31 % D. 78 % E. 69 % 21. Sodium carbonate can be made by heating sodium bicarbonate. 2NaHCO 3 (s) Na 2 CO 3 (s) + CO 2 (g) + H 2 O(g) Given that ΔH = kj and ΔG = 33.1 kj at 25 C, above what minimum temperature will the reaction become spontaneous under standard state conditions? A. 321 K B. 401 K C. 3.9 K D. 0.4 K E. 525 K 22. For the formation of NO2, Kc = [NO2]2/[NO]2 [O2]. At equilibrium in a 2.50 L container, there are 3.00 mol NO, 4.00 mol O2 and 22.0 mol NO2. The value of Kc is: A B C D E Page 6

10 23. The heat of fusion for napthalene (C10H8) is kj/mol and for sodium is 2.60 kj/mol. The amount of heat that would melt grams of sodium would melt how many grams of napthalene? A g B g C g D g E g 24. A liquid will exhibit a convex meniscus and undergo reverse capillary action whenever A. adhesive interactions are strong compared to cohesive interactions. B. adhesive interactions are weak compared to cohesive interactions. C. the surface tension is greater than the viscosity D. it has a high vapor pressure E. it has a low vapor pressure 25. If the critical point of substance W occurs at a pressure of 1360 torr and a temperature of 120 K, which of the following statements is definitely true: A. W cannot be liquified at 25 C at any pressure B. W is liquid at room temperature and one atmosphere of pressure C. W is a non-permanent gas D. W is a solid at room temperature and one atmosphere of pressure E. None of these 26. Based on the phase diagram for substance Z below, which of the following statements is incorrect? A. Z cannot be liquefied at the pressure of (4). B. Solid and gaseous Z will coexist at equilibrium under the conditions of (3). C. Liquid and gaseous Z will coexist at equilibrium under the conditions of (2). D. Starting at the conditions of (3), increasing the temperature at constant pressure will cause solid Z to undergo fusion. E. Liquid Z is more dense than solid Z under the conditions of (5). Page 7

11 27. The following reactions occur at 500 K. Arrange them in order of increasing tendency to proceed to completion (least greatest tendency). A. 3 < 2 < 1 B. 3 < 1 < 2 C. 1 < 2 < 3 D. 2 < 1 < 3 E. 2 < 3 < Henry s Law constants for aqueous solutions at 25 C are molal/mmhg for N2 and molal/mmhg for O2. Determine the solubility of nitrogen (N2) in water under an atmospheric pressure of 760 torr, assuming that air is 80% N2 and 20% O2, by mole. A m B m C m D m E m 29. Consider the following reaction at equilibrium: The equilibrium is displaced to the left if: A. some sulfur dioxide is added B. some sulfur trioxide is removed C. the pressure is raised D. the temperature is raised E. none of these 30. For the reaction: ΔH = 126 kj/mol and ΔS = J/mol K at 377 C. What is Keq? A B C D E Page 8

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FACULTY OF SCIENCE MID-TERM EXAMINATION 2 MARCH 18, :30 TO 8:30 PM CHEMISTRY 120 GENERAL CHEMISTRY

FACULTY OF SCIENCE MID-TERM EXAMINATION 2 MARCH 18, 2011. 6:30 TO 8:30 PM CHEMISTRY 120 GENERAL CHEMISTRY Examiners: Prof. B. Siwick Prof. A. Mittermaier Dr. A. Fenster Name: Associate Examiner: A. Fenster