U.S. National Chemistry Olympiad

1994 National Test

Notes


1. Which solid(s) will react with a 1 M strong acid to produce a gas?

Solids
I. CaCO3
II. K2SO4
III. Mg

(A) I only
(B) II only
(C) I and III only
(D) I, II, and III

2. Transition metals typically have all of these characteristics except

(A) forming colored components.
(B) showing a variety of oxidation states.
(C) possessing one or more unpaired electrons as individual atoms.
(D) having low melting points in the elemental state.

3. When equal volumes of 1 M Na2SO4 and 1 M solutions of two different ions are mixed, a precipitate forms with one of the ions but not with the other. The two ions could be

(A) Ba2+ and Mg2+
(B) Al3+ and Zn2+
(C) Cu2+ and K+
(D) Pb2+ and Ba2+

4. When 1 M sulfuric acid is added to sodium chloride, the gas given off is

(A) HCl
(B) H2S
(C) Cl2
(D) SO2

5. When 25.5 g of HCl are dissolved in 90.0 g of H2O, what is the mole fraction of HCl in the resulting solution?

Molar Mass of HCl is 36.5 g mol¯1

(A) 0.123
(B) 0.139
(C) 0.221
(D) 0.283

6. In an experiment to determine the molar mass of an unknown solid acid, the acid is titrated with a standard solution of NaOH. Which procedural error(s) will result in a molar mass that is too low?

Procedural Mistakes
I. using a NaOH solution that has absorbed CO2
II. rinsing the buret with distilled H2O rather than NaOH solution before titrating
III. allowing some of the solid acid to stick to the side of the flask

(A) I only
(B) III only
(C) I and II only
(D) II and III only

7. Which air pollutant is colored?

(A) CO
(B) NO2
(C) SO2
(D) SO3

8. When 4.50 g of Fe2O3 is reduced with excess H2 in a furnace, 2.60 g of metallic iron is recovered. What is the percent yield?

Fe2O3 + 3 H2 ---> 2 Fe + 3 H2O

The Molar Mass of Fe2O3 is 159.7 g mol¯1

(A) 82.6
(B) 70.0
(C) 57.8
(D) 31.5

9. Which of these compounds contains the greatest percentage of nitrogen?
Molar Mass
C6H3N3O7 229 g mol¯1
CH4N2O 60.1 g mol¯1
LiNH2 23.0 g mol¯1
Pb(N3)2 291 g mol¯1

(A) C6H3N3O7
(B) CH4N2O
(C) LiNH2
(D) Pb(N3)2

10. Analysis shows a certain compound to be 37.51% C, 4.17% H, and 58.32% O. What is its empirical formula?

(A) C3H4O4
(B) C4H5O5
(C) C5H7O6
(D) C6H8O7

11. A sample of a compound of xenon and fluorine contains molecules of a single type; XeFn, where n is a whole number. If 9.03 x 1020 of these XeFn molecules have a mass of 0.311 g, what is the value of n?

(A) 2
(B) 3
(C) 4
(D) 6

12. The combustion reaction of C3H8O in O2 is represented by this equation.

2 C3H8O + 9 O2 ---> 6 CO2 + 8 H2O

When 3.00 g C3H8O and 7.38 g O2 are combined, how many moles of which reagent remain?

The Molar Mass of C3H8O is 60.1 g mol¯1

(A) 0.0056 mol O2
(B) 0.024 mol C3H8O
(C) 0.24 mol O2
(D) 0.18 mol C3H8O

13. The Freezing-Point Depression Constant, Kf of H2O is 1.86 °C m¯1

If 0.0500 mol of dichlorotetrammineplatinum (IV) chloride, [Pt(NH3)4Cl2]Cl2, completely ionizes in 500. g of water, what is the expected freezing point of the solution?

(A) - 0.186 °C
(B) - 0.372 °C
(C) - 0.558 °C
(D) - 0.930 °C

14. Formaldehyde is oxidized by dichromate ions in acid solution to give formic acid as represented by this equation.

3 HCOH + Cr2O72¯ + 8 H+ ---> 3 HCOOH + 2 Cr3+ + 4 H2O

If 15.62 mL of 0.125 M K2Cr2O7 exactly react with 25.00 mL of formaldehyde solution, what is the molarity of the formaldehyde?

(A) 0.00585 M
(B) 0.0260 M
(C) 0.0781 M
(D) 0.234 M

15. If 0.25 mol of a gas has a mass of 4.0 g and occupies a volume of 5.6 L at standard temperature and pressure, what would be the mass and volume of 0.50 mol of this gas at standard temperature and 2.0 atm pressure?

(A) 4.0 g and 5.6 L
(B) 4.0 g and 11 L
(C) 8.0 g and 5.6 L
(D) 8.0 g and 11 L

16. If 34.0 mL of O2 are collected over H2O at 740 mmHg and 25 °C, what will be the volume of dry O2 at 0 °C and 760 mmHg?

Vapor Pressure at 25 °C of H2O is 23.8 mmHg

(A) 29.4 mL
(B) 30.3 mL
(C) 32.0 mL
(D) 33.1 mL

17. What is the molar mass of an ideal gas if a 0.622 g sample of this gas occupies a volume of 300. mL at 35 °C and 789 mmHg?

(A) 44.8 g mol¯1
(B) 48.9 g mol¯1
(C) 50.5 g mol¯1
(D) 54.4 g mol¯1

18. Which is true about equal volumes of CH4 and O2 at 20 °C and 1 atm pressure?

(A) The CH4 sample has a mass that is one-half that of the O2 sample.
(B) The number of O2 molecules is twice as large as the number of CH4 molecules.
(C) The average kinetic energy of the O2 molecules is one-half that of the CH4 molecules.
(D) The average velocity of the O2 molecules is one-half that of the CH4 molecules.

19. Which of these substances has the highest critical temperature?

(A) H2
(B) HCl
(C) H2O
(D) CO2

20. What are the correct labels for the areas designated I and II in this phase diagram?

(A) I - solid; II - liquid
(B) I - solid; II - gas
(C) I - liquid; II - gas
(D) I - gas; II - solid

21. The enthalpy of vaporization of a liquid can be obtained from the straight line obtained from which of these plots?

(A) vapor pressure vs. T
(B) vapor pressure vs. 1/T
(C) natural log vapor pressure vs. T
(D) natural log vapor pressure vs. 1/T

22. For which reaction, carried out at standard conditions, would both the enthalpy and entropy changes drive the reaction in the same direction?

(A) 2 H2(g) + O2(g) ---> 2H2O(l) ΔH = -571.7 kJ
(B) 2 Na(s) + Cl2(g) ---> 2 NaCl(s) ΔH = - 822.0 kJ
(C) N2(g) + 2 O2(g) ---> 2 NO2(g) ΔH = 67.7 kJ
(D) 2 NH2(g) ---> N2(g) + 3 H2(g) ΔH = 92.4 kJ

23. 168.00 J of energy are added to a sample of gallium initially at 25.0 °C, the temperature rises to 38.0 °C. What is the volume of the sample?
Data for Gallium, Ga
specific heat 0.372 J g¯1 °C¯1
density 5.904 g cm¯3

(A) 2.38 cm3
(B) 4.28 cm3
(C) 14.1 cm3
(D) 31.0 cm3

24.
Data for Ethanol, C2H5OH
molar heat of vaporization 39.3 kJ mol¯1
boiling point 78.3 °C

Use these data to calculate ΔS for the vaporization of one mole of ethanol.

(A) 0.502 kJ mol¯1 °C¯1
(B) 1.99 kJ mol¯1 °C¯1
(C) 112 J mol¯11
(D) 502 J mol¯1 °C¯1

25. The combustion of ammonia is represented by this equation.

4 NH3(g) + 5 O2(g) ---> 4 NO(g) + 6 H2O (g) ΔH=- 905 kJ
Enthalpy of formation data
NO2(g) 90.4 kJ mol¯1
H2O(g) - 241 kJ mol¯1

What is the enthalpy of formation for NH3(g)?

(A) - 499 kJ mol¯1
(B) - 46.1 kJ mol¯1
(C) 184 kJ mol¯1
(D) 277 kJ mol¯1

26. For which reaction is ΔH most nearly equal to ΔE?

(A) C(s) + O2(g) ---> CO2(g)
(B) N2(g) + 2 O2(g) ---> 2 NO2(g)
(C) 2 H2O2(l) ---> 2 H2O(l) + O2(g)
(D) C2H4(g) + 3 O2(g) ---> 2 CO2(g) + 2 H2O(l)

27. For the reaction,

4 NH3(g) + 5 O2(g) ---> 4 NO(g) + 6 H2O(g)

which relationship is true?

(A) d[NH3]/dt = d[NO]/dt
(B) -d[O2]/dt = d[NO]/dt
(C) -4d[NH3]/dt = 6d[H2O]/dt
(D) -1/4d[NH3]/dt = 1/6d[H2O]/dt

28. These data were obtained in the study of the reaction between pyridine (C5H5N) and methyl iodide (CH3I)

Initial Concentrations, M  
  C5H5N CH3I Initial Rate, M s¯1
Exp. 1 1.0 x 10¯4 1.0 x 10¯4 7.5 x 10¯7
Exp. 2 2.0 x 10¯4 2.0 x 10¯4 3.0 x 10¯6
Exp. 3 2.0 x 10¯4 4.0 x 10¯4 6.0 x 10¯6

based on these data , what is the experimental rate law?

(A) Rate = k[C5H5N]1[CH3I]1
(B) Rate = k[C5H5N]2[CH3I]0
(C) Rate = k[C5H5N]0[CH3I]2
(D) Rate = k[C5H5N]1[CH3I]2

29. The half-life for the first-order conversion of cyclobutane to ethylene,

C4H8(g) ---> 2 C2H4(g)

is 22.7 at a particular temperature. How many seconds are needed for the partial pressure of cyclobutane to decrease from 100mmHg to 10 mmHg?

(A) 52.0 s
(B) 75.4 s
(C) 90.0 s
(D) 227 s

30. The activation energy for a particular reaction is 83.1 kJ mol¯1. By what factor will the rate constant increase when the temperature is increased from 50.0 °C to 60.0 °C?

(A) 10.0
(B) 2.53
(C) 0.929
(D) 0.935

31. What will be the effect of increasing the temperature of reactants which are known to undergo an endothermic reaction? The effect will be to

(A) Increase both the rate of the reaction and the magnitude of the equilibrium constant
(B) Increase the rate of the reaction and decrease the magnitude of the equilibrium constant
(C) Decrease the rate of the reaction and increase the magnitude of the equilibrium constant
(D) Increase the rate of the reaction but have no effect on the equilibrium constant

32. What is the value of the equilibrium constant, K, for a reaction for which ΔG° = - 5.20 kJ at 50 °C?

(A) 0.144
(B) 0.287
(C) 6.93
(D) 86.4

33. The [product] / [reactant] ratio for the particular reaction is increased with increases in both temperature and pressure. What can you correctly conclude?

I. The reaction is exothermic
II. The volume of the products is greater than the volume of the reactants.

(A) I only
(B) II only
(C) both I and II
(D) neither I nor II

34. Carbon monoxide gas reacts with hydrogen gas at elevated temperatures to form methanol according to this equation.

CO(g) + 2 H2(g) <===> CH3OH (g)

when 0.4 mol of CO and 0.30 mol of H2 are allowed to reach equilibrium in a 1.0 L container, 0.060 mol of CH3OH are formed. What is the value of Kc?

(A) 0.50
(B) 0.98
(C) 1.7
(D) 5.4

35. The reaction of N2 and O2 to give NO2 with an equilibrium constant of K1 is represented by this equation.

N2(g) + 2 O2(g) <===> 2 NO2(g)

when the equilibrium constant, K2, for the reaction;

NO2(g) <===> 1/2 N2(g) + O2(g)

is expressed in terms of K1, the correct expression is;

(A) K2 = -K1
(B) K2 = -1/2 K1
(C) K2 = 1/K11/2
(D) K2 = K11/2

36. Given these acids and their Ka values, which sequence lists the corresponding anions in order of increasing base strength?

Equilibrium values, Ka
HC2H3O2 1.8 x 10¯5
HCN 6.2 x 10¯10
HF 7.2 x 10¯4
(A) CN¯,C2H3O2¯, F¯
(B) C2H3O2¯, CN¯ , F¯
(C) F¯, C2H3O2¯, CN¯
(D) F¯, CN¯, C2H3O2¯

37. Which hydroxides are expected to be amphoteric in aqueous solution

I. Al(OH)3 II. Ca(OH)2
III. NaOH IV. Zn(OH)2
(A) I only
(B) III only
(C) I and IV only
(D) I, II, and IV only

38. What is the pH of the solution that is 0.015 M in benzoic acid, C6H5COOH, and 0.030 M in sodium benzoate, Na+C6H5COO¯? (Ka for benzoic acid is 6.5 x 10¯5)

(A) 6.01
(B) 4.49
(C) 3.89
(D) 3.01

39. A weak monoprotic acid (25.0 mL of a 0.10 M solution ) is titrated to a phenolphthalein end-point with a 0.10 M solution of NaOH. Which statement about this titration is true?

(A) Volume required will be less than 25.0 mL
(B) The pH at the equivalence point will be above seven.
(C) The solution will be colorless after 27.0 mL of NaOH has been added.
(D) The [H+] = [OH¯] at the equivalence point.

40. When dissolved in water to give 0.10 M solution, which salt will produce the solution with the lowest pH?

(A) NH4Cl
(B) CaCl2
(C) NH4C2H3O2
(D) Ca(C2H3O2)2

41. A 0.15 M solution of a weak acid is found to be 1.3 % ionized. What is its Ka?

(A) 1.3 x 10¯2
(B) 2.0 x 10¯3
(C) 1.1 x 10¯3
(D) 2.5 x 10¯5

42. Which equation represents an acid-based reaction according to the Lewis definition but not according to the Brönsted-Lowry definition?

(A) NH3(g) + HCl(g) --> NH4Cl(s)
(B) HF(aq) + OH¯(aq) --> H2O(l) + F¯(aq)
(C) Al(H2O)63+ (aq) --> Al(H2O)5(OH)2+(aq) + H+(aq)
(D) Cu2+(aq) + 4 NH3(aq) --> Cu(NH3)42+(aq)

Use these data to answer questions 43 and 44.

Standard Reduction Potentials
Cr3+ (aq) + 3e¯ ---> Cr (s) - 0.74 V
Co2+ (aq) + 2e¯ ---> Co (s) - 0.28 V

43. Which statement is true about a voltaic cell contructed using the half-cells above? Assume 1 M concentration.

(A) Electron flow in the external circuit will be from chromium to cobalt.
(B) Chromium will be the cathode.
(C) As the reaction proceeds, the cobalt will undergo oxidation.
(D) In the salt bridge connecting the half cells, anions will move toward the cobalt.

44. The standard potential for the cell described in question 43 will be

(A) -1.02 V
(B) 0.46 V
(C) -0.64 V
(D) 1.8 x 1017

45. What would be the value of Keq for an electrochemical cell which has an E° value of 0.34 V and involves the transfer of three electrons? (Assume T = 25°C)

(A) 5.6 x 10¯18
(B) 39.7
(C) 5.6 x 105
(D) 1.8 x 1017

46. How many minutes will be required to deposit 1.00 g of chromium metal from an aqueous CrO3 solution using a current of 6.00 amperes?

(A) 186
(B) 30.9
(C) 15.4
(D) 5.15

47. When an aqueous solution of NaBr is electrolyzed, all of these products are formed except

(A) Br2(l)
(B) H2(g)
(C) OH¯(aq)
(D) Na(s)

48. Which electronic transition occurs with the greatest release of energy in a hydrogen atom?

(A) n = 2 --> n = 3
(B) n = 3 --> n = 9
(C) n = 9 --> n = 3
(D) n = 3 --> n = 2

49. For which element could the last electron have the quantum numbers n = 4, l = 2?

(A) Ca (Z = 20)
(B) Ti (Z = 22)
(C) Si (Z = 32)
(D) Rh (Z = 45)

50. Which atom in its ground state contains the greatest number of unpaired electrons ?

(A) S (Z = 16)
(B) P (Z = 15)
(C) Si (Z = 14)
(D) Al (Z = 13)

51. Which of these isoelectronic species will be the largest?

(A) S2¯
(B) Cl¯
(C) K+
(D) Ca2+

52. When these elements are arranged in order of increasing melting point, the correct order is

(A) Ca, Cr, K
(B) Ca, K, Cr
(C) Cr, K, Ca
(D) K, Ca, Cr

53. Which process will release the greatest quantity of energy?

(A) O(g) ---> O+(g) + e¯
(B) O+(g) ---> O2+(g) + e¯
(C) O¯(g) + e¯ ---> O2¯(g)
(D) O(g) + e¯ ---> O¯(g)

54.

Element Electron Configuration
W 1s2 2s2 2p4
X 1s2 2s2 2p6 3s1
Y 1s2 2s2 2p6 3s2
Z 1s2 2s2 2p6 3s2 3p3

Which pair of elements would form a compound with 1 : 2 stoichiometry?

(A) W and X
(B) W and Y
(C) X and Y
(D) Y and Z

55. According to valence bond theory, what hybrid orbital are used by the central atom in SF4?

(A) sp3
(B) dsp2
(C) dsp3
(D) d2sp3

56. What is the maximum number of covalent bonds that can be formed by an element with the outer electron configuration of 3s2 3p4?

(A) 2
(B) 3
(C) 4
(D) 6

57. In which pair of species do the central atoms have the same geometry?

(A) NH3, NO3¯
(B) BF3, NO3¯
(C) BF3, ClF3
(D) NH3, ClF3

58. Use the given bond energies and this equation to calculate the H-F bond energy in kJ mol¯1

Bond Energies, kJ mol¯1
H2 436
F2 135

H2(g) + F2(g) ---> 2 HF (g) ΔH = - 541 kJ

(A) 1130
(B) 565
(C) 48
(D) 24

59. Which set of species is arranged in order of increasing O-N-O bond angle?

(A) NO2¯, NO2, NO2+
(B) NO2, NO2¯, NO2+
(C) NO2+, NO2, NO2¯
(D) NO2, NO2+, NO2¯

60. That fact that Pt(NH3)2Cl2 exists in two different isomeric forms offers evidence that its geometry is

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

Free Response

1. A magnesium / aluminum alloy reacts with aqueous HCl to give hydrogen gas and aqueous magnesium ions.

a) Write balanced equations for the reaction of HCl with aluminum and with magnesium.
b) When a 0.105 g sample of an Al-Mg alloy is reacted completely with an excess of HCl, 116 mL of hydrogen gas is collected over water at 20 °C and 742 mm Hg at the number of moles of hydrogen gas. (The vapor pressure of H2O = 17.5 mm Hg at 20 °C.)
c) Determine the percentages of Al and Mg in the alloy.

2. A solution is prepared by mixing 50.0 mL of 0.20 M NH3(aq) with 50.0 mL of 0.30 M NH4Cl(aq). (Kb for NH3 is 1.8 x 10¯5 )

a) Write the equilibrium expression for the reaction of NH3 with water.
b) Calculate the pH of this solution.
c) Calculate the pH of this solution after the addition of 100. mL of 0.020 M HCl.
d) Calculate the pH of the original solution after it has been diluted with 100. mL of distilled water.

3. Consider the following chemical reaction at 25 °C;

2 CH3Cl (g) + 4 O2(g) ---> 2 CO2(g) + 3 H2O(g) + Cl2O(g)

for which ΔH° = - 1270.5 kJ and ΔS° = - 29.54 J mol¯11. These absolute entropies (S°) are given.

CH3Cl(g) 234.47 J mol¯11 H2O(g) 188.72 J mol¯11
CO2(g) 213.63 J mol¯11 Cl2O(g) 266.10 J mol¯11
a) Calculate the numerical value of the absolute entropy, S°, for O2(g).
b) Account for the relatively small magnitude of the ΔS° value for this reaction in terms of the properties of the substances involved.
c) Calculate the numerical value for ΔG° for this reaction.
d) If the water present as a product were formed as a liquid rather than a gas,
i. Predict what would happen to the values of ΔH° and ΔS° and explain your reasoning.
ii. Explain why it would be more difficult to predict the effect this change would have on the value of ΔG°.

4. Many metals corrode when placed in contact with water containing O2 from the atmosphere. This process can be attributed to an electrochemical reaction in which oxygen undergoes reduction to form H2O according to the half-reaction;

O2(g) + 4 H+ + 4 e¯ ---> 2 H2O
E° = 1.23 V

a) Write the Nernst equation for this half-reaction.
b) Calculate the potential for this reaction under typical atmospheric conditions. Assume that air has a pressure of 1.0 atm and contains 21% oxygen. Assume also that air pollutants produced by the combustion of fossil fuels have lowered the pH of the water to 4.50.
c) The reduction potential, E°, for Fe2+ + 2 e¯ ---> Fe is -0.44 V. Combine the information in (b) with this reduction of Fe with the aqueous O2.
i. Write a balanced equation for the reaction of Fe with aqueous O2.
ii. Calculate Ecell. (Assume [Fe2+] = 1.0 M)
d) Calculate the value of ΔG that corresponds to the cell potential in (c).
e) Describe qualitatively how you would expect the cell potential to be affected by the following conditions and explain your reasoning.
i. [Fe2+] << 1.0 M when the iron is initially placed in the water.
ii. Efforts to lower the emission of air pollutants have increased the pH above 5.

5. Write the net equations for the following reactions. Use appropriate ionic and molecular formulas for the reactants and products and omit formulas for all organic substances. You need not balance the equations. All reactions occur in the aqueous solutions unless otherwise indicated.

a) Hydrochloric acid is added to solid zinc sulfide.
b) Solutions of barium chloride and phosphoric acid are mixed.
c) Phosphorus is burned in air.
d) An acidic solution of potassium permanganate is added to one of the iron(II) sulfate.
e) Solid sodium acetate is added to water.
f) A solution of hydrogen peroxide is warmed gently.
g) Excess concentrated ammonia is added to a copper(II) chloride solution.

6. Tetrafluoroethylene dimerizes according to the equation;

2 C2F4(g) ---> C4F8(g)

This reaction was followed over a period of time and these data were collected.

time in seconds 0 10 30 75 110
[C2F4] (M) 0.500 0.402 0.289 0.177 0.136
a) Briefly describe how you would determine the order of this reaction and its rate constant, from the data above. (Calculations are not necessary. Just describe how you would treat the data and interpret the results.)
b) Outline how you could determine the activation energy for this reaction. Describe the experiments that should be conducted and how their results should be treated.

7. Consider the related species, SO3 and SO32¯.

a) Draw an electron dot structures for each, including resonance forms if necessary.
b) Use VSEPR theory to predict the geometry and the O-S-O bond angle of each species.
c) Identify the hybrid orbitals involved for each species according to the Valence-Bond Theory.
d) Compare the S-O bond lengths expected for each species and explain your reasoning.

8. The solubilities of the three compounds shown below were determined in water, diethyl ether, and ethanol.

  Solubilities of Each Compound in
Compound H2O C2H5OC2H5 C2H5OH
X i. s. s.
Y vs. i. s.
Z s. s. s.

(Key : s = soluble, vs = very soluble, I = insoluble.)

a) Use the solubility data to identify each of the compounds by matching the Roman Numerals (I, II, III) with the letters (X, Y, Z). Explain your reasoning.
b) Account for the fact that all three compounds are soluble in ethanol whereas neither of the other solvents dissolve all three compounds.
c) Identify the compound you would expect to be the most soluble in benzene, C6H6, and explain your reasoning.