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U.S. National Chemistry Olympiad

1992 National Test

Notes


1. Assuming a diamond is pure carbon, how many carbon atoms are present in a 1.75 carat diamond? 1 carat = 200 mg.

(A) 4.38 x 1017
(B) 5.71 x 1021
(C) 1.75 x 1022
(D) 2.11 x 1023

2. Elemental analysis results obtained for cortisone, an anti-inflammatory agent, are: 69.98% C, 7.83% H , and 22.19% O. What is the empirical formula of cortisone?

(A) C4H6O
(B) C18H22O3
(C) C20H25O4
(D) C21H28O5

3. How many grams of SO2 can be produced from 15 g of CS2 by complete combustion in oxygen?

CS2 + 3 O2 ---> CO2 + 2 SO2

(A) 6.3
(B) 13
(C) 25
(D) 650

4. How many grams of zinc completely react with 50.0 mL of 0.0600 M HCl?

Zn(s) + 2 HCl(aq) ---> ZnCl2(aq) + H2(g)

(A) 0.196 g
(B) 0.0981 g
(C) 0.294 g
(D) 0.0491 g

5. Consider the reaction

2 MoCl5 + 5 Na2S ----> 2 MoS2 + 10 NaCl + S

How many electrons are involved in each half-reaction for each atom of elemental sulfur that is produced?

(A) 1
(B) 2
(C) 3
(D) 4

6. What volume of CO2 at 20 °C and 700. mmHg must be added to a 500. mL container containing N2 at 25 °C and 600. mmHg to give a mixture of gases having a pressure of 900. mmHg at 20 °C?

(A) 220 mL
(B) 330 mL
(C) 440 mL
(D) 660 mL

7. Hydrogen gas reduces Ag2O to silver metal and forms steam in the process. How many liters of H2O(g) are produced at 125 °C and 1.0 atm pressure when 1.0 g of Ag2O is reduced by hydrogen gas?

(A) 0.070 L
(B) 0.14 L
(C) 0.30 L
(D) 0.96 L

8. Which of the following gases would be expected to deviate the most from ideal gas behavior?

(A) Ar
(B) O2
(C) CH4
(D) NH3

9. What is the ratio of the rates of effusion of H2O(g) to D2O(g)? (D = the 1-H-2 isotope of hydrogen)

(A) 0.900
(B) 0.950
(C) 1.05
(D) 1.11

10. What formula would be expected for a binary compound formed between strontium and nitrogen?

(A) SrN
(B) Sr2N
(C) SrN3
(D) Sr3N2

11. Coke is utilized in the manufacture of iron from its ore. What is the purpose of the coke?

(A) an oxidizing agent
(B) a reducing agent
(C) a fuel
(D) both a and c

12. Calcium carbonate, obtained from limestone, would NOT be found in which of the following products?

(A) soil additives to increase soil pH
(B) an antacid tablet formulation
(C) mild abrasive cleaners
(D) water softeners

13. Ionic compounds are NOT usually characterized by:

(A) relatively high heats of fusion and melting points.
(B) conductivity of the melt.
(C) conductivity of the solution if the compound is soluble.
(D) random arrangements of ions, resulting in amorphous solids.

14. Which list is arranged in order of increasing atomic size?

(A) H, K, Li, As, Cs
(B) H, Li. K, As, Cs
(C) H, Li, As, K, Cs
(D) H, As, Li, K, Cs

15. How many electrons occupy energetically equivalent d orbitals in the Ti atom?

(A) 1
(B) 2
(C) 5
(D) 10

16. Which of the following sets contains only linear molecules?

(A) H2S, CO, CO2
(B) BF3, Cl2, O2
(C) CO2, HCN, O2
(D) SO2, HCN, CO2

17. What is the approximate H-N-H bond angle in the ammonia molecule?

(A) 180
(B) 120
(C) 109
(D) 90

18. What is the hybridization and the approximate H-O-C- bond angle for the indicated oxygen atom?

(A) sp, 180
(B) sp2, 120
(C) sp3, 109
(D) d2sp3, 90

19. Which pairs of compounds will form the strongest hydrogen bonds with each other?

(A) C2H5OH and CH3OCH3
(B) HOCH2CH2OH and H2O
(C) HOCH2CH2OH and CH3OH
(D) CH3OCH3 and H2O
20. Which species exhibits only single bonding?
(A) CO2
(B) CH4
(C) CN¯
(D) C6H6

21. What set of quantum numbers is NOT possible?

  n l ml ms
(A) 2 1 1 1/2
(B) 2 1 -1 1/2
(C) 3 2 2 1/2
(D) 3 3 -3 1/2

22. Which statement is correct?

(A) The C-Cl bonds in CCl4 are polar, and CCl4 molecules are polar.
(B) The C-Cl bonds in CCl4 are polar, but CCl4 molecules are nonpolar
(C) The C-Cl bonds in CCl4 molecules are polar
(D) The C-Cl bonds in CCl4 molecules are nonpolar

23. Heisenberg's Uncertainly Principle states that there is a limit to the accuracy at which we can determine the:

(A) position of a particle
(B) momentum of a particle
(C) position and momentum of a particles simultaneously
(D) acceleration of a particle

24. What is the wavelength of a photon of energy 2.6 x 10¯17 J?

(A) 1.1 x 10¯11 cm
(B) 7.6 x 10¯7 cm
(C) 2.4 x 10¯7 cm
(D) 4.0 x 10¯5 cm

25. A solution of 5.00 g of unknown X dissolved in 20.0 g benzene freezes at -4.52 °C. Benzene normally freezes at 5.48 °C and its kf is 5.12 °C/m. What is unknown X?

(A) para-dichlorobenzene, C6H4Cl2
(B) naphthalene, C10H8
(C) menthol, C10H20O
(D) some other compound

26. What is the standard free-energy charge at 25 °C for each mole of chromium oxidized by permanganate ion:

Cr(s) + MnO4¯ (aq, 1M in acid) ----> Mn2+ (aq, 1 M) + Cr3+
Reduction Potentials
Cr3+ ---> Cr E° = +0.74 V
MnO4¯ ---> Mn2+ (in acid) E°= +1.51 V

(A) -6.51 x 105 J
(B) -1.09 x 106 J
(C) -1.11 x 106 J
(D) -3.26 x 106 J

27. Consider a concentration cell based on copper electrodes in Cu2+ solution. When the cell is tested using solutions of 0.15 M and 1.0 M concentrations, the voltage is found to be 0.0243 V. If the concentration of the Cu2+ solution in the oxidatiuon compartment is doubled, what is the voltage?

(A) 0.0486 V
(B) 0.0332 V
(C) 0.0155 V
(D) 0.0122 V

28. In a charged battery, the individual cells are composed of lead, lead(IV) oxide, and sulfuric acid solution. As the battery discharges, lead(II) sulfate and water are formed. Which statement best describes the reaction occurring when the battery is used to start the car?

(A) The lead(IV) oxide acts as a cathode an forms lead.
(B) The lead(IV) oxide acts as an anode and forms lead(II) sulfate.
(C) The lead acts as a cathode and forms lead(IV) oxide.
(D) The lead acts as an anode and forms lead(II) sulfate.

29. Nitric acid (HNO3) is a strong oxidizing agent. Which of the following species is least likely to be produced when nitric acid reacts with a strong reducing agent such as zinc metal?

(A) NO
(B) N2O
(C) N2O5
(D) NO2

30. The standard reduction potentials at 25 °C are:
Half reaction  
Mg2+(aq) + 2e¯ ---> Mg(s) E° = -2.37V
Sr2+(aq) + 2e¯ ---> Sr(s) E° = - 2.89V

What is the equilibrium constant for the following reaction at 25 °C ?

Sr(s) + Mg2+(aq) ----> Sr2+ (aq) + Mg(s)

(A) 2.6 x 10¯18
(B) 1.6 x 10¯9
(C) 6.3 x 108
(D) 3.9 x1017

31. Which of the following is true when EDTA is used as metal complexing agent?

(A) It forms 1:1 stoichiometric complexes with large equilibrium constants.
(B) It forms complexes selectively with particular metal ions, regardless of pH.
(C) All complexes are in 2:1 ratio with small equilibrium constants.
(D) Only transition metals complex strongly with EDTA.

32. Consider the reaction:

N2O4(g) <===> 2 NO2(g).

If 0.020 mol of N2O4(g) is placed in a 500-ml container and allowed to reach equilibrium, [N2O4] = 0.0055 M. What is Kc for the reaction?

(A) 0.870
(B) 12.5
(C) 6.27
(D) 0.138

33. The following reaction occurs in a container of variable volume at 25 °C. DH° = -92 kJ. Which of the statements is true?

N2(g) + 3 H2(g) <===> 2 NH3(g)

(A) Kp and Kc are equal for the reaction under this set of conditions.
(B) If the volume of the container decreases, the reaction will shift to the right and Kc will increase.
(C) If the external pressure on the container decreases, the reaction will shift to the left and Kc will remain unchanged.
(D) High pressure and temperature favor maximum conversion of reactants to products.

34. Which element listed is most easily oxidized?

(A) Mg
(B) Fe
(C) Co
(D) Au

35. When 2.75 mol HI(g) are placed into a 1.00-L container at 25 °C and allowed to dissociate according to the equation:

2 HI (g) <===> H2(g) + I2(g)

The final concentration of H2(g) is found to be 0.275 M. What is the Kc for the reaction?

(A) 0.0275
(B) 0.0100
(C) 0.0123
(D) 0.0156

36. Which of the following acids dissociates to the greatest extent in aqueous solution?

(A) acetic acid
(B) chloroacetic acid
(C) dichloracetic acid.
(D) trichloroacetic acid.

37. What is the pH of 2.00 x 10¯3 M Ba(OH)2 solution?

(A) 2.70
(B) 3.40
(C) 11.30
(D) 11.60

38. When 1.00 g of a strong acid is dissolved in 100 mL of water the pH of the solution is 0.908. Assuming no change in volume when the solution is formed, calculate the molar mass of the unknown acid.

(A) 20 g mol¯1
(B) 36 g mol¯1
(C) 63 g mol¯1
(D) 81 g mol¯1

39. What is the initial concentration of a sample of acetic acid if the pH of the solution is 4.85? (Ka acetic acid= 1.8 x 10¯5

(A) 0.100 M
(B) 1.1 x 10¯5
(C) 1.4 x 10¯5
(D) 2.5 10¯5

40. What is the pH of a buffer prepared using a 2:1 ratio of acetic acid (pKa = 4.74) to acetate ion?

(A) pH = 4.74
(B) pH > 4.74
(C) pH < 4.74
(D) The pH cannot be calculated without knowing the molarities of the components.

41. The Ksp values for Ca(OH)2 and Mg(OH)2 are 6.5 x 10¯6 and 7.1 x 10¯12 respectively. Choose the best pH to selectively separate a mixture that is 0.10 M in both Ca2+ and Mg2+.

(A) 2.0
(B) 6.0
(C) 10.0
(D) 13.0

42. The Ksp for BaSO4 is 1.1 x 10¯10 and that for BaSeO4 is 2.8 x 10¯11. A 1.0 M solution of BaCl2 is added slowly to a solution that is 1.0 x 10¯4 M in sodium sulfate, Na2SO4 and 1.0 x 10¯4 M in sodium selenate, Na2SeO4. What percentage of one anion has precipitated at the point which the second anion just begins to precipitate?

(A) Under 10%
(B) About 25%
(C) About 75%
(D) Over 90 %

43. What will NOT increase the rate of the following reaction?

NH3(g) + HCl(g) ----> NH4Cl(s)

(A) increase in temperature
(B) increase in pressure
(C) increase in [NH3(g)]
(D) increase in volume

44. A proposed mechanism for the reaction 2 NO(g) + Br2(g) ---> 2 NOBr(g) is as follows:

NO(g) + Br2(g) ----> NOBr2 (g) slow
NO(g) + NOBr2 (g) ----> 2NOBr(g) fast

The rate law is:

(A) rate = k[NO][Br2]
(B) rate = k[NO]2[Br2]
(C) rate= k[NO][NOBr2]
(D) rate = k[NO]2

45. What is the activation energy of a reaction if the rate constant increases by a factor of 10 when the temperature is raised from 303 K to 333 K?

(A) 64 kJ
(B) 46 kJ
(C) 33 kJ
(D) 30 kJ

46. The half-life of radioactive 55Cr is 1.8 hours. The delivery of a sample of this isotope from the reactor to your laboratory requires about 10.8 hours. What is the minimum amount of such materials that should be shipped in order that you receive 1.0 mg of 55Cr?

(A) 130 mg
(B) 64 mg
(C) 32 mg
(D) 11 mg

47. Which of the following hydroxides is amphoteric?

(A) KOH
(B) Ba(OH)2
(C) Mg(OH)2
(D) Al(OH)3

48. A white solid is known to be one of the following: K2C2O4, CaCO3, NH4NO3, or Ca(NO3)2. The substance is only slightly soluble in water. It dissolves readily in HCl, releasing a gaseous product. What is the compound?

(A) K2C2O4
(B) CaCO3
(C) NH4NO3
(D) Ca(NO3)2

49. Why is carbon monoxide toxic to humans?

(A) It reacts with water to form carbonic acid.
(B) It is strongly basic.
(C) It is a strong oxidizing agent.
(D) It binds strongly to hemoglobin.

50. Which process will be accompanied by a positive DG° at 25 °C?

(A) 2 Cl(g) ---> Cl2(g)
(B) HClO4(aq) + OCl¯(aq) ---> ClO4¯(aq) + HOCl(aq)
(C) 2 NaCl(l) ----> 2Na(l) + Cl2(g)
(D) Cl2(aq) + 2 Br¯(aq) ---> 2 Cl¯(aq) + Br2(aq)

51. When ammonia is added to a solution, a brown precipitate forms and the supernantant liquor turns deep blue. Which of the following best explains this phenomena?

(A) The solution contains Cu2+ and Ni2+
(B) The solution contains Fe3+ and Cu2+
(C) The solution contains Ni2+ and Fe3+
(D) The solution contains Fe2+ and Fe3+

52. What is one of the starting materials for the copolymer nylon?

(A) acrylonitrile
(B) 1,6-hexanediamine
(C) styrene
(D) vinyl chloride

53. Mercury bateries, like those used in electric watches, furnish a voltage of 1.35V. If the overall oxidation-reduction equation taking place is

Zn°(s) + HgO(s) + H2O(l) <===> Zn(OH)2(s) + Hg°(l)

the anode reaction must be

(A) HgO(s) + H2O (l) + 2e¯ <===> Hg°(l) + 2 OH¯(aq)
(B) Zn°(s) + 2 OH¯ (aq) <===> Zn(OH)2(s) + 2e¯
(C) Hg°(s) + 2 OH¯ (aq) <===> HgO(s) + H2O(l) + 2e¯
(D) Zn(OH)2(s) + 2e¯ <===> Zn°(s) + 2 OH¯(aq)

54. Which will be about equally soluble in water and in 0.1 M HCl?

(A) NaClO4
(B) MgCO3
(C) PbCl2
(D) Al2O3

55. The ion-product constant for water, Kw, has a value of 9.6 x 10¯14 at 60 °C. These date indicate the autoionization reaction of water has:

  DD
(A) + +
(B) + -
(C) - +
(D) - -

56. Consider the combustion of propane in a movable piston apparatus:

C3H8(g) + 5 O2(g) -----> 3 CO2(g) + 4 H2O(l)

Under these condition, what are the signs of q and w in DE = q + w?

  q w
(A) + +
(B) + -
(C) - +
(D) - -

57. A 1.00 g sample of n-hexane, C6H14, undergoes complete combustion with excess O2 in a bomb calorimeter having a calorimeter constant of 10.3 kJ C¯1. The temperature of the calorimeter rises from 22.64 °C to 29.30 °C. What is the DE for this reaction in kJ mol¯1 of n-C6H14?

(A) -68.6 kJ mol¯1
(B) -8.92 x 102 kJ mol¯1
(C) -6.73 x 103 kJ mol¯1
(D) -5.91 x 103 kJ mol¯1

58. Estimate the carbon-carbon triple bond strength in acetylene, C2H2, given the heat of combustion of acetylene is -1.26 x 103 kJ mol¯1 and the following bond energies.
Bond Type Bond Energy
in kJ mol¯1
C-H 414
C=O 803
O-H 464
O=O (O2) 498

(A) 841 kJ
(B) 807 kJ
(C) 309 kJ
(D) 1260 kJ

59. From the following data,

C(graphite) + O2(g) ---> CO2(g) DH° = -393.5 kJ
H2(g) + 1/2 O2(g) ----> H2O(l) DH° = -285.8 kJ
2 C2H6(g) + 7 O2(g) -----> 4 CO2(g) + 6 H2O(l) DH° = -3120 kJ

the enthalpy of formation of C2H6(g) is

(A) -6410 kJ
(B) -168 kJ
(C) -84.4kJ
(D) -2440 kJ

60. The balanced equation for the reduction of the nitrate anion by the Fe(II) ion in an acidic solution is:

(A) 3 Fe2+(aq) + NO3¯ (aq) + 4 H+ (aq) <===> 3 Fe3+(aq) + NO(g) + 2 H2O(l)
(B) Fe2+(aq) + NO(g) + 8 H+(aq) <===> Fe3+ (aq) + 2NO(g) + 4 H2O(l)
(C) 2 Fe2+(aq) + 2 NO3¯(aq) + 4 H+(aq) <===> 2 Fe3+(aq) + 2 NO(g) + 4 H2O(l)
(D) 3 Fe3+(aq) + NO(g) + 2 H2O(l) <===> 3 Fe2+(aq) + NO3¯(aq) + 4 H+(aq)

61. The geometry of a molecule is best described in terms of the locations of the atomic nuclei. What geometries are possible for compounds whose bonding can be described using sp3 hybrid orbitals?

(A) Trigonal planer or bent
(B) Tetrahedral, bent, or trigonal pyramidal
(C) Square planar, tetrahedral, or trigonal planar
(D) Trigonal bipyramidal or seesaw

62. What is the shape of the BrO3¯ ion?

(A) bent
(B) trigonal planar
(C) trigonal pyramidal
(D) tetrahedral

63. What is the hybrid orbital set used for bonding by Xe in XeF2?

(A) sp2
(B) sp3
(C) dsp3
(D) d2sp3

64. Which substance should be most soluble in benzene, C6H6?

(A) NH3
(B) NaCl
(C) CH3OH
(D) CCl4

65. Which of the radioactive decay products has the shortest average lifetime in the atmosphere?

66. What is the charge on the central metal ion in Ca3[Fe(CN)6]2?

(A) 0
(B) 1
(C) 2
(D) 3

67. Important commercial extraction processes utilize carbon dioxide above its critical temperature. Under these comditions

(A) carbon dioxide is no longer fluid.
(B) carbon dioxide is known commonly as dry ice
(C) one cannot condese the gas to a liquid by applying pressure
(D) CO2 dissociates into C and O2

68. The anti-cancer drug cis-platin has the formula Pt(NH3)2Cl2. There is another isomer, tans-platin, that is not medically active. What is the shape of cis-platin?

(A) tetrahedral
(B) octahedral
(C) square planar
(D) trigonal bipyramidal

69. How many structural isomers of heptane exist?

(A) 9
(B) 8
(C) 6
(D) 4

70. What is one of the products of the addition of HBr to 2-butene?

(A) 1-bromobutane
(B) 2-bromobutane
(C) 1,2-dibromobutane
(D) 2,3-dibromobutane

Free Response

1. The percentages of sodium carbonate and sodium chloride in a mixture of them is to be determined by reacting the mixture with an excess of hydrochloric acid. When a 4.00-g sample of this mixutre is added to 0.100 L of 2.85 M HCl, a gas is produced.

(A) Write a balanced equation to represent the reaction that is responsible for the production of the gas.
(B) After drying, the gas is found to occupy a volume of 335 mL measured at 21 °C and 724 mmHg. Calculate the number of moles of gas in this sample.
(C) Calculate the percentage of sodium carbonate and sodium chloride in the mixture.
(D) Calculate the number of moles of hydrochloric acid that remain after the reaction is complete.

2. Benzoic acid, C6H5COOH, is a weak acid (Ka = 6.5 x 10¯5) that is often used as a food preservative, either in the form of the acid or its sodium salt.

(A) Write a chemical equation to represent the ionization of benzoic acid in water.
(B) In a solution prepared by mixing 250 mL of 0.0500 M sodium benzoate and 150 mL of 0.100 M benzoic acid, calculate the initial concentrations of benzoic acid and benzoate ion.
(C) Calculate the [H3O+] for the solution in part (B).
(D) How many moles of sodium benzoate must be added to 0.250 L of 0.100 M benzoic acid in order to produce a buffer solution with a pH of 5.25? (Assume no change in volume.)

3. The enthalpy change for the reaction below is -46 kJ.

OCl2(g) + H2O(g) ----> O2(g) + 2 HCl(g)

(A) Calculate Df for OCl2. (Df for HCl and H2O are -92.3 and -242 kJ mol¯1 respectively.)
(B) Write an equation to represent the formation of OCl2 from O2 and Cl2.
(C) Calculate the bond energy for the O-Cl bond. (The bond energies for O2 and Cl2 are 498 and 243 kJ mol¯1 respectively.)
(D) Account for the large difference between the O2 and Cl2 bond energies.

4. A 500-mL solution of 0.235 M CuCl2 was placed in an electrolytic cell. After electrolyzing between platinum electrodes for a certain time, 3.861 g of copper were deposited at one electrode and chlorine gas was evolved at the other electrode.

(A) At which electrode (anode or cathode) was the coppper deposited?
(B) Calculate the number of moles of copper deposited.
(C) Assuming 100% efficiency, calculate the:
(i) number of Faradays of electricity used.
(ii) length of time required if a current of 1.78 amps was used.
(D) Calculate the [Cl¯] in solution after the electrolysis.

5. Write net equations for each reaction. Use appropriate ionic and molecular formulas for the reactants and products and omit formulas for all ions or molicules that do not take part in a reaction. Write structural formulas for all organic substances. You need not balance the equations.

(A) Sulfuric acid is added to a sodium nitrite solution.
(B) A solution of potassium dichromate is added to a solution of sulfurous acid.
(C) Solid sodium phosphate is added to water.
(D) A solution of concentrated aqueous ammonia is added to solid silver chloride.
(E) Copper metal is added to concentrated nitric acid.
(F) Solutions of barium chloride and potassium carbonate are mixed.
(G) Solutions of iron(III) sulfate and potassium iodide are mixed.
(H) Strontium-90 decays by the loss of a beta particle.

6. Describe procedures that could be carried out in a school laboratory to acomplish each of the following and idicate the results you would expect:

(A) Remove sodium sulfate impurities from lead sulfate.
(B) Identify the halide ion (Cl¯, Br¯, I¯) in a solution of a sodium halide.
(C) Separate a mixture of cyclohexane (b.p. 80.7 °C) and benzene (b.p. 80.1 °C).
(D) Prepare gaseous ammonia from solid ammonium chloride.
(E) Determine X in a solid sample of CuSO4 . X H2O.

7. Ozone, O3, is an important component of the upper atomosphere that decomposes readily to form oxygen, O2. One proposed mechanism for this decomposition is given below:

(A) Write a balanced equation for the decomposition of ozone.
(B) Use the mechanism above to write the rate law for this reation.
(C) How could the initial reate of ozone decomposition be decreased without changing either the concentration of ozone or the temperature?
(D) Nitric oxide, NO, was of much concern a few years ago when it was thought that its production by supersonic transports in the upper atmosphere could promote the decomposition of O3 via its oxidation to NO2. Give two different ways that NO could promote the decomposition of O3 according to the mechanism above.

8. Explain the following observations:

(A) Elemental sulfur is more soluble in methylbenzene than in water, whereas sodium chloride is more soluble in water than in methylbenzene.
(B) The solubilities of gases in water decrease with an increase in temperature, but increase with an increase in temperature.
(C) The boiling points of stright-chain hydrocarbons are generally higher than those of their branched-chain isomers.
(D) Most members of the first transition series exhibit a +2 oxidation shate regardless of their group number whereas nontransition elements exhibit oxidation shates that are common within each group but seldom between groups.
(E) Lithium metal is a stronger reducing agent in aqueous solution than sodium metal; even though the ionization energy of Li is greater than that of Na.