U.S. National Chemistry Olympiad

1996 National Test

Notes

• and [sigma] are used to indicate the capital Greek letters.
• [square root] applies to the numbers enclosed in parenthesis immediately following
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1. What is the formula for the basic anhydride of Ba(OH)₂?

(A) Ba₂O
(B) BaO
(C) BaO₂
(D) Ba

2. Which element exhibits both +1 and +3 oxidation numbers in its compounds?

(A) B
(B) Be
(C) Sn
(D) Ti

3. Which element is obtained commercially by electrolysis?

(A) aluminum
(B) iron
(C) oxygen
(D) sulfur

4. A 0.1 M solution of a certain cation will form a precipitate with 0.1 M solutions of all of these anions; OH¯, CO₃²¯, SO₄²¯. Which cation fits this description?

(A) Ba²⁺
(B) Fe²⁺
(C) Mg²⁺
(D) Pb²⁺

5. Each of these solutions is added to a mixture of aqueous sodium bromide and chloroform,CHCl₃. Which one will give a positive test for aqueous bromine when the solutions are vigorously mixed?

(A) NaCl solution
(B) NaI solution
(C) NaI₃ solution
(D) chlorine water

6. Oxalic acid dihydrate, H₂C₂O₄ ^. 2 H₂O(s) is often used as a primary standard to standardize sodium hydroxide solutions. Which of these facts are reasons to choose this substance as a primary standard?

I. It is diprotic.
II. It is a stable compound that can be weighed directly in air.
III. It is available in a pure form.

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

7. When FeCl₃ is ignited in an atmosphere of pure oxygen,this reaction takes place.

4 FeCl₃(s) + 3 O₂(g) ---> 2 Fe₂O₃(s) + 6 Cl₂(g)

If 3.0 mol of FeCl₃ are ignited in the presence of 2.0 mol of O₂ gas, how much of which reagent is present in excess and therefore remains unreacted?

(A) 0.33 mol FeCl₃ remain unreacted
(B) 0.67 mol FeCl₃ remain unreacted
(C) 0.25 mol O₂ remain unreacted
(D) 0.50 mol O₂ remain unreacted

8. How many grams of SbF₃ are needed to produce a gram of Freon-12, CCl₂F₂, according to the reation represented by this equation?

3 CCl₄ + 2 SbF₃ ---> 3 CCl₂F₂+ 2 SbCl₃

(A) 0.667g
(B) 0.986g
(C) 1.50g
(D) 2.22g

9. A self-contained breathing apparatus uses potassium superoxide, KO₂, to convert the carbon dioxide and water in exhaled air into oxygen, as shown by the equation.

4 KO₂(s) + 2 H₂O(g) + 4 CO₂(s) ---> 4 KHCO₃(s) + 3 O₂(g)

How many molecules of oxygen gas will be produced from the 0.0468 g of carbon dioxide that is exhaled in a typical breath?

(A) 4.8 x 10²⁰
(B) 6.4 x 10²⁰
(C) 8.5 x 10²⁰
(D) 1.9 x 10²¹

10. Magnetite, Fe₃O₄, can be converted into metallic iron by heating with carbon monoxide as represented by this equation.

Fe₃O₄(s) + 4 CO(g) ---> 3 Fe(s) + 4 CO ₂(g)

How many kilograms of Fe₃O₄ must be processed in this way to obtain 5.00 kg of iron if the process is 85% efficient?

(A) 6.92 kg
(B) 8.15 kg
(C) 20.8 kg
(D) 24.4 kg

11. What is the molality of an aqueous sucrose solution that is 25.0% sucrose, C₁₂H₂₂O₁₁ by mass?

(A) 0.585 m
(B) 0.731 m
(C) 0.975 m
(D) 1.17 m

12. A student finds that 31.26 mL of a 0.165 M solution of barium hydroxide, Ba(OH)₂, solution is required to just neutralize 25.00 mL of a citric acid, H₃C₆H₅O₇, solution. What is the concentration of the H₃C₆H₅O₇ solution?

(A) 0.413 M
(B) 0.309 M
(C) 0.206 M
(D) 0.138 M

13. Which Group III element is expected to have physical and chemical properties that are the least similar to the other elements in that family?

(A) B
(B) Al
(C) Ga
(D) Tl

14. A pure gas that is 14.4% hydrogen and 85.6% carbon by mass has a density of 2.5 g L¯¹ at 0 °C and 1 atm presure. What is the molecular formula of the gas?

(A) CH₂
(B) C₂H₄
(C) C₄H₈
(D) C₆H₁₂

15. The vapor pressure of most substances increase with temperature as depicted by this curve. Which way of plotting these data is expected to give a straight line?

(A) 1/P versus T
(B) P versus 1/T
(C) In P versus T
(D) In P versus 1/T

16. If 0.10 mL of liquid water are introduced into a 1.0 L flask at 25 °C, how many moles of water are in the vapor phase when equilibrium is established?

(A) 1.28 x 10¯³
(B) 5.56 x 10¯³
(C) 1.53 x 10¯²
(D) 4.46 x 10¯²

17. Sodium chloride, NaCl, usually crystallizes in a face-centered cubic lattice. How many Cl¯ ions are in contact with any single Na⁺ ion?

(A) 4
(B) 6
(C) 8
(D) 12

18. When solid NH₄NO₃ is dissolved in water at 25 °C, the temperature of the solution decreases. What is true about the signs of DH and DS for this process?

	DH	DS
(A)	-	+
(B)	-	-
(C)	+	+
(D)	+	-

19. If the equilibrium constant for a reaction is very small (for example, 1 x 10¯²⁰), what would the value of DG be expected to be?

(A) a small positive number
(B) a large positive number
(C) a small negative number
(D) a large negative number

20. Use the bond energies in the table to determine DH for the formation of hydrazine, N₂H₄, from nitrogen and hydrogen according to this equation:

N₂(g) + 2 H₂(g) ---> N₂H₄(g)

(A) DH = 711 kJ
(B) DH = - 98 kJ
(C) DH = + 98 kJ
(D) DH = + 711 kJ

21. Use the given heats of formation to calculate the enthalpy change for this reaction:

B₂O₃(s) + 3 COCl₂(g) ---> 2 BCl₃(g) + 3 CO₂(g)

(A) 694.3 kJ
(B) 354.9 kJ
(C) -58.9 kJ
(D) -3917.3 kJ

22. For which of these processes is the value of DS negative?

I. Sugar is dissolved in water.
II. Steam condenses on a surface.
III. CaCO₃ is decomposed into CaO and CO₂.

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

23. What is the expected value of DG° for this reaction? (E° for the reaction = 0.46 V)

Cu(s) + 2 Ag⁺(aq) ---> Cu²⁺(aq) + 2 Ag(s)

(A) -89 kJ
(B) -44 kJ
(C) -22kJ
(D) 15kJ

24. What is the change in internal energy, DE, for a system that does 70 joules of work as it absorbs 45 joules of heat?

(A) 115 J
(B) 25 J
(C) -25 J
(D) -115 J

25. What are the units of a second-order rate constant?

(A) sec¯¹
(B) mol L¯¹ sec¯¹
(C) mol² L¯² sec¯¹
(D) L mol¯¹ sec¯¹

26. The half life for the radioactive decay of ₃₂P is 14.3 days. How many days would be required for a sample of a radiopharmaceutical containing ₃₂P to decrease to 20% of its initial activity?

(A) 33.2 d
(B) 61.8 d
(C) 71.5 d
(D) 286 d

27. The rates of many chemical reactions double for a ten degree rise in temperature. Which of these factors does not contribute to this change in rate with increasing temperature?

(A) the average kinetic energy of the reactant species.
(B) the number of collisions in a given time.
(C) the number of very energetic species.
(D) the activation energy.

28. The bromination of acetone that occurs in acid solution is represented by this question.

CH₃COCH₃(aq) + Br₂(aq) ---> CH₃COCH₂Br(aq) + H⁺(aq) + Br¯(aq)

These kinetic data were obtained for given reaction concentrations.

Based on these data, what is the rate equation?

(A) Rate = k[CH₃COCH₃][Br₂][H⁺]
(B) Rate = k[CH₃COCH₃][H⁺]
(C) Rate = k[CH₃COCH₃][Br₂]
(D) Rate = k[CH₃COCH₃][Br₂][H⁺]²

29. What is the initial rate of the reaction (in mol L¯¹ sec¯¹) depicted by this graph?

(A) 0.02
(B) 0.01
(C) 0.008
(D) 0.005

30.

2 SO₂(g) + O ₂(g) <===> 2 SO₃(g)

Given that the equilibrium constant for the reaction above has a value of 278 at a particular temperature, what is the value of the equilibrium constant for the following reaction at the same temperature?

SO₃(g) <===> SO₂(g) + 1/2 O₂(g)

(A) 1.3 x 10¯⁵
(B) 1.8 x 10¯³
(C) 3.6 x 10¯³
(D) 6.0 x 10¯²

31.

Cl₂(aq) + H₂S(aq) ---> S(s) + 2 H⁺(aq) + 2 Cl¯(aq)

The rate equation for this reaction is: rate = k[Cl₂][H₂S]

Which of these mechanisms is (are) consistent with this rate equation?

(A) I only
(B) II only
(C) Both I and II
(D) Neither I or II

32. When a sample of NO₂ is placed in a container, this equilibrium is rapidly established.

2 NO₂(g) <===> N₂O₄(g)

If this equilibrium mixture is a darker color at high temperatures and at low pressures, which of these statements about the reaction is true?

(A) The reaction is exothermic and NO₂ is darker in color than N₂O₄.
(B) The reaction is exothermic and N₂O₄ is darker in color than NO₂
(C) The reaction is endothermic and NO₂ is darker in color than N₂O₄.
(D) The reaction is endothermic and N₂O₄ is darker in color than NO₂.

33. When the acids; H₂Se, HBr, and HI, are arranged in order of increasing strength (weakest acid first), which is the correct order?

(A) H₂Se < HBr < HI
(B) HBr < HI < H₂Se
(C) HBr < H₂Se < HI
(D) HI < H₂Se < HBr

34. Which statement is true about the relationship between Brønsted-Lowry (B-L) bases and Lewis bases?

(A) A Lewis base must be a B-L base but a B-L base need not be a Lewis base.
(B) A B-L base must be a Lewis base but a Lewis base need not be a B-L base.
(C) B-L bases and Lewis bases are interchangeable.
(D) There is no relationship between Lewis and B-L bases.

35. A solution of 2.0 M formic acid (HCOOH) is 0.95% ionized. What is the K_a of formic acid?

(A) 1.9 x 10¯²
(B) 1.8 x 10¯⁴
(C) 9.0 x 10¯⁵
(D) 4.5 x 10¯⁵

36. If 0.1 mol of a salt is added to 1.0 L of water, which of these salts is expected to produce the most acidic solution?

(A) NaC₂H₃O₂
(B) NH₄NO₃
(C) CuSO₄
(D) AlCl₃

37. The pK_a values for several acid-base indicators are given in the table. Which indicator should be used in the titration of a weak base with a strong acid?

(A) 2,4-dintrophenol
(B) bromthymol blue
(C) cresol red
(D) alizarin yellow R

38. Which graph best represents the electrical conductivity behavior that occurs when an aqueous solution of acetic acid, HC₂H₃O₂, is titrated with an aqueous solution of sodium hydroxide, NaOH?

39. When this oxidation-reduction equation is correctly balanced, what is the mole ratio of reducing agent to oxidizing agent?

MnO₄¯ + Sn²⁺ + H⁺ ---> Mn²⁺ + Sn⁴⁺ + H₂O

(A) 1:1
(B) 2:1
(C) 4:1
(D) 5:2

40. The solubility of solid silver chromate, Ag₂CrO₄, is determined in three solvents.

I. pure water
II. 0.1 M AgNO₃
III. 0.1 M Na₂CrO₄

Predict the relative solubility of Ag₂CrO₄ in the three solvents.

(A) I = II = III
(B) I < II < III
(C) II = III < I
(D) II < III < I

41. Which combination of reactants will produce the greatest voltage based on these standard electrode potentials?

(A) Cu⁺ and Sn²⁺
(B) Cu⁺ and Cr²⁺
(C) Cu and Sn⁴⁺
(D) Sn⁴⁺ and Cr²⁺

42. The reduction of oxygen gas in 1 M aqueous acid is represented by this equation: (E° = 1.23 V)

O₂(g) + 4H⁺(aq) + 4e¯ ---> 2H₂O(l)

The potential for this reaction at physiological pH, where [H⁺] = 1 x 10¯⁷ and the partial pressure of O₂ is 1 atm, will be closest to which value?

(A) 0.18 V
(B) 0.41 V
(C) 0.82 V
(D) 1.64 V

43. What products are formed during the electrolysis of a concentrated aqueous solution of sodium chloride?

I. Cl₂(g)
II. NaOH(aq)
III. H₂(g)

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

44. A current of 5.00 A is passed through an aqueous solution of chromium(III) nitrate for 30.0 min. How many grams of chromium metal will be deposited at the cathode?

(A) 0.027 g
(B) 1.62 g
(C) 4.85 g
(D) 6.33 g

45. The hydrogen line spectrum provides evidence for the

(A) Heisenberg Uncertainly Principle
(B) wavelike properties of light
(C) diatomic nature of H₂
(D) quantized nature of atomic energy states

46. What is the number of unpaired electrons in a manganese atom (Z = 25) in its lowest energy states?

(A) 1
(B) 3
(C) 5
(D) 7

47. Ions with the electronic structure 1s² 2s² 2p⁶ 3s² 3p⁶ would not be present in which aqueous solution?

(A) NaF(aq)
(B) NaCl(aq)
(C) KBr(aq)
(D) CaI₂(aq)

48. An element with the electron configuration [Xe]4f¹⁴ 5d⁷ 6s², is

(A) an alkaline earth element
(B) a transition element
(C) an inert gas
(D) a rare earth

49. The neutron/proton ratio in an isotope can be increased by the emission of

(A) an electron
(B) a neutron
(C) a gamma ray
(D) a positron

50. The bonds in ozone, O₃, are best represented as

(A) distinct single and double bonds
(B) a single and a double bond that switch positions rapidly
(C) something between a single and a double bond
(D) two double bonds

51. Which pair of substances will have the most similar geometry?

(A) SO₃ and SO₃²¯
(B) SO₃ and CO₃²¯
(C) SO₃ and SO₄²¯
(D) SO₄²¯ and CO₃²¯

52. What hybridization is expected for ClF₃?

(A) sp³
(B) dsp²
(C) dsp³
(D) d²sp³

53. Which bond properties are consistent with one another?

	Bond Order	Bond Length	Vibrational Frequency
(A)	higher	shorter	higher
(B)	higher	longer	lower
(C)	lower	shorter	lower
(D)	lower	longer	higher

54. Which hydrogen halide has the lowest boiling point?

(A) HF
(B) HCl
(C) HBr
(D) HI

55. Which substance has a dipole moment?

(A) CCl₄
(B) CH₂Cl₂
(C) C₂Cl₂
(D) C₂Cl₄

56. What is the correct name for this compound?

(A) 2-ethyl-4, 4-dimethylpentane
(B) 2-methyl-4-t-butylbutane
(C) 2,2,4-trimethylhexane
(D) 1-t-butyl-2-methylbutane

57. How many different structures exist for C₂H₂Cl₂?

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

58. When ethanol is heated with sulfuric acid, the major product is

(A) ethane
(B) ethene
(C) ethyne
(D) ethyl sulfate

59. Many alcohols can be oxidized to aldehydes, and aldehydes can be oxidized to

(A) carboxylic acids
(B) esters
(C) ethers
(D) ketones

60. Which statement about polymers is not true?

(A) They are macromolecules
(B) They have simple repeating units
(C) They have very high molecular masses
(D) They are all made by chemists

1. (12%) This graph shows the results of mixing 50.0 mL portions of bleach (0.637 M aqueous NaOCl) with the specified volumes of 0.455 M aqueous Na₂SO₃ and sufficient water to bring the total volume of each solution to 200 mL.

a) If the sulfur-containing product of this reaction is SO₄²¯, give the formulas of two possible chlorine-containing products. Explain your reasoning.
b) Account for the appearance of the graph and discuss the information that can be obtained from it.
c) Calculate the number of moles of NaOCl in 50.0 mL of this bleach solution.
d) Determine the number of moles of Na₂SO₃ that react stoichiometrically with 50.0 mL of bleach. Explain your answer.
e) Determine the OCl¯ to SO₃²¯ mole ratio from your results in parts c and d.
f) Give the formula for the chlorine-containing product formed and the number of electrons transfered per OCl¯ ion. Explain your answer.

2. (16%) A solution of a diprotic acid H₂A (K_a1 = 1.5 x 10¯⁴, K_a2 = 8.0 x 10¯⁷) containing 0.010 moles H₂A per 100 mL is titrated with a very concentrated solution of KOH so that the volume can be assumed to remain constant during the titration.

a) Sketch the titration curve expected for this titration, showing clearly the equivalence points and the moles of titrant required to reach these points.
b) Calculate the [H⁺] of the solution after the addition of the following numbers of moles of KOH.

i. 0.000 mol
ii. 0.0050 mol
iii.0.010 mol
iv. 0.015 mol
v. 0.020 mol
vi. 0.025 mol

3. (12%) Consider this reaction for the combustion of ethanol. The DH° = - 1234.7 kJ and the DS° = 251.5 J K¯¹ at 25°C

C₂H₅OH(l) + 3O₂(g) ---> 2CO₂(g) + 3H₂O(g)

Also consider these data for thermodynamic values at 25°C.

a) Calculate the value of DH°_f for C₂H₅OH(l).
b) Calculate S° for O₂(g).
c) For this reaction at 25°C, calculate both DG° and the equilibrium constant, K_eq, for the combustion reaction.
d) If the water formed as a product in this reaction appeared as a liquid rather than as a gas, how would DH for the reaction be expected to change? Explain your answer.

4. (9%) Nickel-cadmium (nicad) alkaline batteries are currently very popular because they maintain a constant potential and are rechargeable. The relevant half-cell reactions for such batteries are given in this table.

a) Write a balanced equation for the process that produces electricity ina nicad battery and specify the half-reaction that occurs at the anode.
b) Calculate the potential expected for a nicad battery.
c) If such a battery produces a current of 50 milliamps for a period of 6.00 hours, how many electrons will be transfered?
d) What mass of cadmium will undergo oxidation (or reduction) during the process in part (c)?

5. (14%) Write net equations for each of these reactions. Use appropriate ionic and molecular formulas for the reactions and products and omit formulas for all ions or moleculs that do not take part in a reaction. Write structural formulas for all organic substances. You need not balance the equations. All reactions occur in aqueous solution unless otherwise indicated.

a) A piece of calcium is added to water.
b) Concentrated hydrochloric acid is added to manganese(IV) oxide.
c) Solutions of lead acetate and dilute sulfuric acid are mixed.
d) Sodium cyanide is added to water.
e) Silver is added to dilute nitric acid.
f) Excess aqueous sodium hydroide is added to aqueous aluminum nitrate.
g) A mixture of acetic acid and excess methanol is refluxed in the presence of a catalyst.

6. (11%) Sulfur can form many different compounds, including H₂S, SCl₂, SCl₄, SF₂, SF₄, and SF₆.

a) Account for each of these observations.

i. There are no known compounds of sulfur in an odd atomic ratio with another element, such as SF₃.
ii. The highest known hydride of sulfur contains just two hydrogen atoms per sulfur.
iii. Fluorine combines with sulfur to form SF₂, SF₄, and SF₆; chlorine combines with sulfur to form SCl₂ and SCl₄ but not SCl₆.

b) Write an electron dot structure for SF₄.
c) Sketch the structure of SF₄, give the F-S-F bond angles and identify the hybrid orbitals used by sulfur in this compound.

7. (14%) The conductivity of several aqueous solutions was tested, giving these results.

a) Account for the conductivity results reported for 0.10 M aqueous solutions of CuCl₂, Cu(CH₃CO₂)₂, and H₂S.

Two additional tests are to be performed, this time on mixtures of solutions.

b) For each of these reaction mixtures, answer these questions.

i. Write a net ionic equation, giving state symbols for all species.
ii. Describe any visual changes expected in mixing these solutions.
iii. Predict the relative conductivity expected for the final solution that results from the mixture of the two solutions. Explain your answer.

8. (12%) Account for each of these observations about the rates of chemical reactions and the factors that affect them. Provide properly labeled diagrams as needed to support your answers.

a) The orders of reactants in a rate equation must be determined by experiment and are often different from the coefficients in the balanced equation for a reaction.
b) The starting materials for many reactions can mixed with no observations results until something is done to initiate the reaction.
c) The rates of most chemical reactions increase with an increase in temperature.