2009 Chem 13 News Exam

UNIVERSITY OF WATERLOO

DEPARTMENT OF CHEMISTRY

14 MAY 2009

TIME: 75 MINUTES

This exam is being written by several thousand students. Please be sure that you follow the instructions below.
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  5. Carefully detach the last page. It is the datasheet.
  6. Now answer the exam questions. Questions are not in order of difficulty. Indicate your choice on the STUDENT RESPONSE sheet by marking one letter beside the question number.
    • Mark only one answer for each question.
    • Questions are all of the same value.
    • There is a penalty (1/4 off) for each incorrect answer, but no penalty if you do not answer.
  7. Take care that you make firm, black pencil marks, just filling the oval.

    Be careful that any erasures are complete—make the sheet white again.

Carefully detach the last page. It is the Data Sheet.

1. In the third period of the elements, how do the atomic radii of the elements vary?

  1. The radii increase steadily from Na to Ar.
  2. The radii increase from Na to Al and decrease from Al to Ar.
  3. There is no regular pattern.
  4. The radii decrease from Na to S and increase from S to Ar.
  5. The radii decrease steadily from Na to Ar.

2. Which of the following compounds has the highest boiling point?

  1. CH3CCl=CClCH3 
    CH3CCl=CClCH3
  2. (CH3)2C=C(CH3)2 
    (CH3)2C=C(CH3)2
  3. CH3C≡CCH3 
    CH3C≡CCH3
  4. ClC≡CCl 
    ClC≡CCl
  5. (CH3)2C=CHCH3 
    (CH3)2C=CHCH3

3. Nitrous acid, HNO2, is a weak acid in water. Which of the following statements concerning NO2- is true?

Ka=7.2×10-4 at 298 K for HNO2.

  1. NO2- is a weak base.
  2. NO2- is a strong base.
  3. NO2 is a weak acid.
  4. NO2- is a strong acid.
  5. NO2- is neither an acid nor a base.

4. How many moles of NaOH should be added to 1.0 L of 0.10 mol L-1 HCOOH(aq) to obtain a solution having a final pH of 4.0 at 298 K? Assume no change in volume. (Choose the closest value.)

Ka=1.8×10-4 at 298 K for HCOOH.

  1. 0.018 mol
  2. 1.8 mol
  3. 0.26 mol
  4. 0.064 mol
  5. 0.0099 mol

5. Which of the following molecules do not form hydrogen bonds amongst themselves?

  1. CH3COOH
  2. H2O2
  3. CH3OCH3
  4. HF
  5. CH3CH2OH

6. The reaction below reaches equilibrium in a closed reaction vessel.

4 HCl(aq) + MnO2(s) ΔH<0 ⇌ Cl2(g) + 2 H2O(l) + Mn2+(aq) + 2 Cl-(aq)

Which of the following actions increases the mass of Cl2(g) in the equilibrium mixture?

  1. adding some MnO2(s)
  2. increasing the temperature
  3. adding some MnCl2(s)
  4. decreasing the volume of the reaction vessel
  5. adding something that precipitates Mn2+

Use the following information and diagram to answer questions 7-10.

A galvanic cell is constructed by placing a strip of zinc into a 1.0 mol L-1 solution of zinc nitrate and a strip of aluminum into a 1.0 mol L-1 solution of aluminum nitrate. The two metal strips are connected to a voltmeter by wires and a salt bridge connects the solutions. (See the diagram below.) The temperature is 25 °C. The following standard reduction potentials apply:

Al3+(aq) + 3e- → Al(s)   E° = -1.67 V

Zn2+(aq) + 2e- → Zn(s)   E° = -0.76 V

A diagram of a galvanic (voltaic) cell setup featuring zinc and aluminum electrodes in their respective nitrate solutions, connected by a salt bridge and a voltmeter. Note: Zn(NO3)2(aq)=1.0 mol L-1; Al(NO3)3(aq)=1.0 mol L-1

7. What is E° for the cell described above?

  1. 0.15 V
  2. 0.91 V
  3. 2.43 V
  4. 3.49 V
  5. 6.53 V

8. In the cell described above, where does reduction occur?

  1. at the aluminum electrode
  2. at the zinc electrode
  3. at the voltmeter
  4. in the salt bridge
  5. in the aluminum nitrate solution

9. Considering the standard reduction potentials given in the box on the right, which of the following is the strongest reducing agent under standard conditions?

  1. Al(s)
  2. Al3+(aq)
  3. Zn(s)
  4. Zn2+(aq)
  5. impossible to determine

10. What is Ecell equal to when the cell described in the box reaches equilibrium at 25 °C?

  1. -2.43 V
  2. +5.62 V
  3. 0 V
  4. 1.06 V
  5. none of the above

11. A compound is 54.6% C, 36.2% O and 9.2% H by mass. What is the empirical formula of the compound?

  1. CH2O
  2. C2H4O
  3. C3H6O2
  4. C4H4O
  5. C6H6O

12. What is the pH of a 1.25×10-7 mol L-1 HCl(aq)?

  1. 6.90
  2. 6.74
  3. 7.00
  4. 6.67
  5. less than 6.67

13. Which of the following statements is true?

  1. A single covalent bond consists of a single delocalized electron.
  2. For a bond formed between a given pair of atoms, the bond dissociation energy increases as the bond order decreases.
  3. The bond dissociation energy for a C=C bond is twice that of a C-C bond.
  4. A polar covalent bond results from the transfer of one or more electrons from one atom to another.
  5. none of the above

14. How many isomers are there for C5H12?

  1. one
  2. two
  3. three
  4. four
  5. more than four

15. The enthalpy change for the reaction below is ΔH = -58 kJ (per mole of N2O4 formed).

2NO2(g) + 2 I-(aq) ⇌ 2 SO42-(aq) + I2(s)

If k1 and k-1 are the rate constants for the forward and reverse reactions, respectively, and Kc is the equilibrium constant for the reaction as written, then what effect does increasing the temperature have on the values of k1, k-1, and Kc?

  1. k1 increases, k-1 decreases, Kc increases
  2. k1 increases, k-1 increases, Kc increases
  3. k1 increases, k-1 increases, Kc decreases
  4. k1 increases, k-1 decreases, Kc decreases
  5. k1 decreases, k-1 decreases, Kc decreases

16. Consider the thermochemical equations below.

C2H4(g) + 3 O2(g) → 2 CO2(g) + 2 H2O(l) ΔH° = -1411 kJ (per mol C2H4)

C2H4(g) + H2O(l) → C2H5OH(l) ΔH° = -44 kJ (per mol C2H4)

What is ΔH° for the following reaction? All the answers below are for the combustion of one mole of C2H5OH.

C2H5OH(l) + 3 O2(g) → 2 CO2(g) + 3 H2O(l)

  1. -1089 kJ
  2. 632 kJ
  3. -1455 kJ
  4. -1733 kJ
  5. -1367 kJ

17. A 10.0-L gas cylinder contains neon gas with a measured pressure of 5.50 atm at 298 K. The 10.0-L cylinder is then connected to an empty gas cylinder of unknown volume, and the neon gas expands to fill both cylinders. If the final pressure is found to be 3.76 atm at 298 K, then what is the volume of the second cylinder?

  1. 14.6 L
  2. 6.52 L
  3. 10.0 L
  4. 4.63 L
  5. 9.26 L

18. Which of the following correctly describes what happens when aqueous solutions of ammonium carbonate, (NH4)2CO3, and potassium bromide, KBr, are mixed?

  1. Br- neutralizes NH4+.
  2. K2CO3(s) precipitates.
  3. HBr is formed.
  4. NH4Br(s) precipitates.
  5. none of the above

Use the information and diagram below to answer questions 19-21.

A 40.0-mL sample of a weak monoprotic acid, HA, is titrated with 0.20 mol L-1 NaOH(aq). The titration curve is shown below.

A titration curve graph showing the pH change as sodium hydroxide solution is added to a substance.  Four specific points on the curve are highlighted with their corresponding pH values: pH=2.86 at approximately 0 mL NaOH. pH=4.89 at approximately 15 mL NaOH. pH=8.91 at approximately 30 mL NaOH. pH=12.40 at approximately 40 mL NaOH.

19. For the titration described above, which of the following is true at the equivalence point?

  1. [HA]=[Na+]
  2. [A-]=[HA]
  3. [Na+]=[A-]
  4. [H+]=[OH-]
  5. [A-]=[H+]

20. Based on the titration curve above, what was the concentration of the original sample solution (i.e. before the titration started)?

  1. 0.63 mol L-1
  2. 0.15 mol L-1
  3. 0.24 mol L-1
  4. 0.067 mol L-1
  5. 0.20 mol L-1

21. Based on the titration curve above, what is the ionization constant (Ka) for the acid, HA?

  1. 10-12.40
  2. 10-8.91
  3. 10-4.89
  4. 10-2.86
  5. 10+2.86

22. How many unpaired electrons are there in the nickel (Ni) atom in its ground state electron state?

  1. 5
  2. 4
  3. 6
  4. 2
  5. 0

23. Equal volumes of 0.1 mol L-1 HCl(aq) and 0.1 mol L-1 HF(aq) are titrated in separate experiments with 0.1 mol L-1 NaOH(aq). Which of the following would be equal for both titrations?

  1. the initial pH (i.e. the pH before any NaOH is added)
  2. the pH when half the acid has been neutralized (i.e. the pH at the half-neutralization point)
  3. the pH at the equivalence point
  4. the volume of NaOH required to reach the equivalence point
  5. none of the above

24. For the reaction below, Kc = 7.8×108. What is the equilibrium concentration NH3 when 1.00 mol each of Zn(NO3)2 and NH3 are dissolved in water to make 1.0 L of solution?

Zn2+(aq) + 4 NH3(aq) ⇌ Zn(NH3)42+(aq)

  1. 0 mol L-1
  2. 1.3×10-9 mol L-1
  3. 0.75 mol L-1
  4. 0.25 mol L-1
  5. 4.5×10-3 mol L-1

25. Which of the following molecules is polar?

  1. CS2
  2. N2O
  3. CCl4
  4. PF5
  5. SO3

26. Consider the compounds NaCl, AgCl and CO2 in terms of their solubilities in water. Which of these compounds exhibits an increase in solubility if the temperature is lowered and the pressure is increased?

  1. NaCl only
  2. AgCl only
  3. CO2 only
  4. NaCl and AgCl
  5. NaCl, AgCl and CO2

27. When a 1.00 mol L-1 solution of M2+(aq) is electrolyzed with a current of 2.5 amperes for 0.2 hours, 0.485 g of M(s) are deposited. What is the identity of M? (Note: 1 ampere = 1 C s-1)?

  1. Cr
  2. Rh
  3. Na
  4. Mg
  5. Ag

28. Iron (III) oxide, Fe2O3, reacts with hydrochloric acid to produce only water and a salt. What is the formula of the salt?

  1. FeCl3
  2. FeCl2
  3. FeCl
  4. Fe2Cl3
  5. FeCl

29. The unbalanced chemical equation for the oxidation of Zn by NO3- is given below. The reaction occurs in aqueous basic solution.

Zn + NO3- → Zn(OH)42- + NH3

How many moles of NO3- are required to oxidize exactly one mole of Zn?

  1. 1 mol
  2. ¼ mol
  3. 4 mol
  4. 8 mol
  5. ⅛ mol

30. Two students each made four measurements of the mass of an object. Their results are shown in the table below

Measurements: Student A Student B
  51.6 g 50.1 g
  50.8 g 49.6 g
  52.2 g 51.0 g
  50.2 g 49.4 g
Average: 51.3 g 50.0 g

If the exact mass of the object is 51.0 g, then which of the following statements is true?

  1. Student A's results are more accurate and more precise.
  2. Student B's results are more accurate and more precise.
  3. Student A's results are more accurate but less precise.
  4. Student B's results are more accurate but less precise.
  5. The two sets of results are equally precise.

31. Which of the following compounds displays the greatest ionic character in its bonds?

  1. NO2
  2. CO2
  3. H2O
  4. NH3
  5. F2O

32. What is the oxidation state of oxygen in FOCN? The molecular structure of FOCN is shown below.

• Lewis structure shows a chain of four atoms: fluorine (F), oxygen (O), carbon (C), and nitrogen (N). F atom is on the far left has a single bond to the oxygen atom and three lone pairs of electrons (six dots) around it. O has a single bond to the F atom and a single bond to C with two lone pairs of electrons (four dots) around it. C has a single bond to O and a triple bond to the N atom and has no lone pairs of electrons. N has a triple bond to C and has one lone pair of electrons (two dots) around it.
  1. zero
  2. +2
  3. -2
  4. +1
  5. -1

33. Experiment shows that in the formamide molecule, H2NCHO, the H-N-H angle is 119° and the N-C-O angle 124° Which of the following structures is an acceptable structure for H2NCHO and is consistent with the experimentally-bond angles?

  1.  
    Structure A
  2. Structure B
  3. Structure C
  4. Structure D
  5. Structure E

34. Which of the following best describes the bonding in the N2 molecule?

  1. one σ bond and one π bond
  2. two σ bonds and one π bond
  3. two π bonds
  4. three σ bonds
  5. one σ bond and two π bonds

35. When building up the electron configuration of a neutral atom, which orbital fills immediately after the 5s orbital?

  1. 4d
  2. 4f
  3. 5p
  4. 6s
  5. 5d

36. What is the formula of perovskite?

Perovskite is a mineral containing Ca, O and Ti. The smallest repeating unit in the structure of perovskite is shown below. (There is a single titanium atom at the centre of the cube.) By considering the total number of atoms of each type that lie inside the cube below, determine the formula of perovskite.

Structure of perovskite. Ca, O, and Ti atoms are identified within the structure.
  1. CaOTi
  2. CaO2Ti
  3. Ca8O8Ti
  4. Ca2O3Ti
  5. CaO3Ti

37. Consider the Lewis structure shown below for the polyatomic ion, EOF₂⁻. The central atom, E, is an unidentified element. Which of the following atoms could E represent?

Lewis structure for EOF2 2−. E has a single bond with F and O, a double bond with another F atom, and two lone pairs of electrons.
  1. oxygen (O)
  2. sulfur (S)
  3. bromine (Br)
  4. nitrogen (N)
  5. xenon (Xe)

38. Consider the following reaction mechanism.

(CH₃)₃C⁺ + N₃⁻ → (CH₃)₃CN₃
(CH₃)₃CBr ↔ (CH₃)₃C⁺ + Br⁻

According to this mechanism, (CH₃)₃C⁺ is

  1. a reaction product
  2. a reaction intermediate
  3. an activated complex
  4. a catalyst
  5. a Lewis base

39. What is the final temperature when 100.0 mL of water at 90.0 °C and 200.0 mL of water at 10.0 °C are mixed? Assume no heat is lost to the surroundings. Choose the closest value.

Properties of water
density = 1.0 g mL⁻¹
specific heat = 4.18 J g⁻¹ °C⁻¹
heat of vaporization = 2260 J g⁻¹

  1. 40 °C
  2. 50 °C
  3. 70 °C
  4. 80 °C
  5. 100 °C

40. Which of the following is present in the greatest number in a dilute sulfuric acid (H₂SO₄) solution?

  1. H₂SO₄ molecules
  2. HSO₄⁻ ions
  3. SO₄²⁻ ions
  4. H⁺ ions
  5. OH⁻ ions

CHEM 13 NEWS EXAM 2010 DATA SHEET

DETACH CAREFULLY

Periodic table for reference during the exam. Click one of the interactive periodic tables in the text to use as a reference.

Additional interactive periodic tables

Constants

NA = 6.022 x 1023 mol-1
R = 0.08206 atm L K-1 mol-1 = 8.3145 kPa L K-1 mol-1 = 8.3145 J K-1 mol-1

Kw = 1.0 \times 10^{-14} (at 298 K)
F = 96 485 C mol-1

Conversion factors

1 atm = 101.325 kPa = 760 torr = 760 mm Hg
0oC = 273.15 K

Equations

PV = nRT
k t1/2 = 0.693
pH = pKa + log ([base]/[acid])

The quadratic formula: x equals the fraction with numerator negative b plus or minus the square root of b squared minus 4ac, and denominator 2a

CHEM 13 NEWS EXAM © 2010 UNIVERSITY OF WATERLOO

Chem 13 News Exam 2009 Answer key

  1. E. The radii decrease steadily from Na to Ar.

  2. B. (CH3)2C=C(CH3)2 

  3. A. NO2- is a weak base.

  4. D. 0.064 mol

  5. C. CH3OCH3

  6. E. adding something that precipitates Mn2+

  7. B. 0.91 V

  8. B. at the zinc electrode

  9. A. Al(s)

  10. C. 0 V

  11. B. C2H4O

  12. B. 6.74

  13. E.none of the above

  14. C. three

  15. C. k1 increases, k−1 increases, Kc decreases

  16. E. −1367 kJ

  17. D. 4.63 L

  18. E. none of the above

  19. C. [Na+ ] = [A− ]

  20. B. 0.15 mol L−1

  21. C. 10−4.89

  22. D. 2

  23. D. the volume of NaOH required to reach the equivalence point

  24. E. 4.5×10−3 mol L−1

  25. B. N2O

  26. C. CO2 only

  27. A. Cr

  28. A. FeCl3

  29. B. ¼ mol

  30. C. Student A’s results are more accurate but less precise.

  31. C. H2O

  32. A. zero

  33. D. Structure D

  34. E. one σ bond and two π bonds

  35. A. 4d

  36. E. CaO3Ti

  37. B. sulfur(S)

  38. B. a reaction intermediate

  39. A. 40 oC

  40. D. H+ ions