U.S. National Chemistry Olympiad: 1990 National Test

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1. In moving from left to right across a period in the periodic table of the elements

(A) ionization energy decreases
(B) atomic radius decreases
(C) electronegativity decreases
(D) electron affinity decreases

2. Which transition is associated with the largest change in energy in the hydrogen atom?

(A) n = 5 to n = 3
(B) n = 2 to n = 1
(C) n = 3 to n = 2
(D) n = 4 to n = 2

3. In which pair are the elements most similar in their chemical properties?

(A) B and N
(B) Li and Fr
(C) Mg and Al
(D) S and Cl

4. A 4.0 g sample of impure Ca(NO₃)₂ was found to contain 0.85 g of calcium. What percentage of Ca(NO₃)₂ was in the original sample? (molar mass of Ca(NO₃)₂ = 164.1 g mol¯¹)

(A) 13 %
(B) 36 %
(C) 64 %
(D) 87 %

5. Aluminum hydroxide, Al(OH)₃, is insoluble in water, but dissolves readily in both acidic and basic solutions. Such behavior is characteristic of

(A) polyprotic behavior.
(B) hydrophilic behavior.
(C) amphoteric behavior.
(D) a buffer.

6. The freezing point of a 1.00 m aqueous solution of HF is found to be -1.91 °C. The freezing point constant is water, k_f, is 1.86 K m¯¹. What is the percent of dissociation of HF at this concentration?

(A) 2.7 %
(B) 5.2 %
(C) 10 %
(D) 30 %

7. Enthapies of formation cannot be measured for many compounds directly, and must be calculated from combustion data. Given the following data, calculate the heat of formation of glucose, C₆H₁₂O₆.

(A) -1272 kJ mol¯¹
(B) +1272 kJ mol¯¹
(C) +2128 kJ mol¯¹
(D) -3488 kJ mol¯¹

8. If the enthalpy of fusion of iodine is +15.5 kJ mol¯¹, and the enthalpy of sublimaiton of iodine is +57.3 kJ mol¯¹, what is the enthalpy of vaporization of iodine?

(A) -41.8 kJ mol¯¹
(B) +41.8 kJ mol¯¹
(C) -72.8 kJ mol¯¹
(D) +72.8 kJ mol¯¹

9. If, for N₂, DH_vap = 5.58 kJ mol¯¹ and DS_vap = 72.1 J mol¯¹ K¯¹, at low temperatures, in what phase will N₂ be at -165 °C?

(A) gas
(B) liquid
(C) solid
(D) at the liquid/gas phase boundary

10. Which of the following molecules contains a central atom which is sp² hybridized?

(A) H₂SO₄
(B) H₂CO₃
(C) ICl₂
(D) H₃CCH₃

11. Which of the following molecules has at least one non-bonding pair of electrons on the central atom?

(A) CHCl₃
(B) HCN
(C) H₂CO
(D) O₃

12. Which of the following statements about ionic compounds is false?

(A) Ionic compounds are hard, brittle solids with high melting points.
(B) Some compounds which exhibit primarily ionic bonding in the solid phase can form covalent bonds in the gas phase.
(C) The lattice energy is the quantity of energy required for 1 mol of a solid ionic substance to be separated completely into gaseous ions well removed from one another.
(D) In general the lattice energy increases as the charge on the anion and cation increase and increases as the size of the anion and cation decrease.

13. The pH of a saturated solution of calcium hydroxide is 12.40. What is K_sp for this salt?

(A) 6.3 x 10¯³⁸
(B) 1.58 x 10¯²⁵
(C) 7.9 x 10¯⁶
(D) 1.58 x 10¯⁵

14. The mass of a nitrogen gas molecule is 14 times that of a hydrogen molecule. What is the hydrogen molecule's velocity, relative to that of the nitrogen molecule (V_N2), if both are at the same temperature?

(A) 0.27 V_N2
(B) 3.7 V_N2
(C) 14 V_N2
(D) 196 V_N2

15. The K_sp of PbBr₂ is 6.3 x 10¯⁶. If 50 mL of 0.020 M Pb(NO₃)₂ are mixed with 50 mL of 0.010 M CaBr₂, which of the following is true?

(A) the solution will not form a precipitate
(B) calcium nitrate will precipitate
(C) PbBr₂ will precipitate and excess Pb²⁺ will remain in solution
(D) PbBr₂ will precipitate and excess Br¯ will remain in solution

16. According to the phase diagram to the right, if the pressure is increased at constant temperature, from point A, what change will occur?

(A) Solid will change to vapor
(B) Liquid will change to vapor
(C) Liquid will change to solid
(D) Solid will change to liquid

17. How many of the following salts will be more soluble in acid solution than in pure water?

CdCO₃, Mn(OH)₂, PbS, PbCl₂

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

18. A 50.0 mL sample of nitrogen gas is collected over water at 25 °C and 720 torr. What is the volume in mL of the dry gas at STP? (The vapor pressure of water at 25 °C is 23.8 torr.)

(A) 42.0
(B) 43.4
(C) 57.6
(D) 59.6

19. To which of the following would addition of an equal volume of 0.50 M NaOH lead to a solution having a lower pH?

(A) 0.30 M HCl
(B) water
(C) 0.60 M KOH
(D) 0.40 M NaNO₃

20. Consider the solvent effect on acid dissociation. Would you expect HCl to be weak or strong in the two solvents, water and benzene (C₆H₆)?

(A) strong in both
(B) weak in both
(C) strong in water, weak in benzene
(D) weak in water, strong in benzene

21. The equilibrium constant for the equation below is very small; that is, K << 1.

HX (aq) + Y¯(aq) <===> X¯ (aq) + HY (aq)

Which statement is true?

(A) The reaction goes nearly to completion
(B) HY is a weaker acid than HX
(C) X¯ is a better proton acceptor than Y¯
(D) Y¯ is a stronger base than X¯

22. Consider the reaction 3A (g) + 2B (g) <===> C (g). Initially, only A and C are present, each at a concentration of 0.20 M. At equillibrium, the concentration of B is 0.10 M. What is the value of the equilibrium constant, K_c, for the reaction?

(A) 6
(B) 43
(C) 60
(D) 350

23. Consider the system

NH₄Cl (s) <===> NH₃ (g) + HCl (g)

If the concentration of ammonia gas is tripled, the value of the equilibrium constant will

(A) triple
(B) increase, buy by more than a factor of three
(C) decrease to one-third its value
(D) remain the same

24. The weak base ionization constant (K_b) for hydroxylamine, HONH₂, is 1.1 x 10¯⁸. Which of the following equations best describes its ionization equilibrium?

(A) HONH₂ (aq) + H₂O (l) ---> HONH₃⁺ (aq) + OH¯ (aq)
(B) HONH₂ (aq) ---> NH₂⁺ (aq) + OH¯ (aq)
(C) HONH₂ (aq) + H₂O (l) ---> ONH₂¯ (aq) + H₃O⁺ (aq)
(D) HONH₂ (aq) + H₃O⁺ (aq) ---> HONH₃⁺ (aq) + H₂O (l)

25. Which substance, when added to water, will NOT change the pH?

(A) NaHCO₃
(B) NH₄Cl
(C) KCN
(D) KCl

26. How many O₂ molecules are contained in 2.0 liters of oxygen gas at 27 °C and 3.0 atm pressure?

(A) 1000
(B) 1.5 x 10²³
(C) 1.5 x 10²⁴
(D) 2.5 x 10²⁴

27. What is the vapor pressure of an isopropanol solution containing 80.0 g of the nonvolatile solute glycerol (C₃H₈O₃, molar mass = 92.0 g mol¯¹) and 212 g is isopropanol (C₃H₈O, molar mass = 60.0 g mol¯¹) at 68 °C? (The vapor pressure of isopropanol at 68 °C is 400 torr.)

(A) 79
(B) 212
(C) 321
(D) 400

28. If 40.0 mL of 0.20 M CH₃COOH are titrated with 0.20 M NaOH, how many mL of base must be added to form a buffer solution with the greatest buffering capacity?

(A) 5.0
(B) 10.0
(C) 20.0
(D) 40.0

29. An important reaction in the production for nitrogen fertilizers is

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

At constant temperature and pressure, what is the maximum number of liters of NO (g) that can be produced from 14.0 L of NH₃ and 16.0 L of O₂?

(A) 12.8
(B) 14.0
(C) 17.5
(D) 20.0

30. Which of the following graphs for an ideal gas, when all other variables are held constant, will NOT yield a straight line?

(A) P versus 1/V
(B) V versus T
(C) n versus T
(D) P versus T

31. What is the equilibrium constant for a reaction that has a value of DG = - 41.8 kJ at 100 °C?

(A) -5.87
(B) 1.4 x 10¯⁶
(C) 13.5
(D) 7.1 x 10⁵

32. How much heat must be removed from a 10.0 g ingot of stainless steel in order to lower its temperature from 60.0 °C to 35.0 °C? (The specific heat of stainless steel is 0.51 J g¯¹ K¯¹.)

(A) 128 J
(B) 179 J
(C) 306 J
(D) 409 J

33. What is the standard molar entropy change, DS, for the synthesis of ammonia, using the following data.

(A) -99.5 J K¯¹1
(B) -130 J K¯¹
(C) +130 J K¯¹
(D) +516 J K¯¹

34. Which of the following reactions is accompanied by a decrease in entropy?

(A) H₂O (l) ---> H₂O (g)
(B) C₂H₂ (g) + 2 H₂ (g) ---> C₂H₆ (g)
(C) NH₄Cl (s) + H₂O ---> NH₄Cl (aq)
(D) H₂ (g) + Cl₂ ---> 2 HCl

35. Trouton's Rule, which states that font face="Symbol">DH_vap / T_bp = 85 J mol¯¹ K¯¹ (a constant) for nonpolar liquids, relates to the entropy change for the vaporization process, DS_vap. Knowing this, how would the value for water differ from this constant?

(A) It would be smaller than 85 J mol¯¹ K¯¹.
(B) It would be larger than 85 J mol¯¹ K¯¹.
(C) It would also be 85 J mol¯¹ K¯¹.
(D) It is impossible to predict.

36. At equilibrium, the Gibbs free energy function of a reaction, DG, has a value

(A) greater than zero.
(B) less than zero.
(C) equal to zero.
(D) which depends on the particular system.

37. An ammonia solution has a density of 0.910 g cm¯³ and is 25.0% NH₃ by mass. What is the molarity of the solution?

(A) 12.1 M
(B) 13.4 M
(C) 14.5 M
(D) 15.5 M

38. Which of the following molecules has a measurable dipole moment?

(A) BrF₃
(B) XeF₄
(C) CF4
(D) PCl₅

39. Which type of crystal structure best describes that present in diamond?

(A) ionic
(B) covalent
(C) metallic
(D) extended molecular

40. Which of the following substances has the highest melting point?

(A) KCl
(B) ClO₂
(C) CaO
(D) BiCl₃

41. Which of the following does not define a covalent bond?

(A) a shared pair of electrons
(B) an overlap of half-filled atomic or hybrid orbitals
(C) increased electron density in the region between atoms of two nonmetals
(D) electrostatic attraction between species in which one or more electrons have been transferred

42. How many equivalent contributing resonance forms does the ion NO₃¯ have?

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

43. Which compound is the anhydride of sulfurous acid?

(A) SO₄
(B) SO₃
(C) SO₂
(D) Na₂SO₄

44. Which is the most accurate description of Na⁺ species in dilute aqueous solution?

(A) the nearest neighbor is a Cl¯ ion
(B) the nearest neighbor is the positive end of the water dipole
(C) the nearest neighbor is the negative end of the water dipole
(D) there are no interactions with any species in the solution

45. Which of the following oxides, at the same concentration when dissolved in water, results in the most acidic solution?

(A) CO₂
(B) B₂O₃
(C) N₂O₅
(D) Li₂O₂

46. Which of the following salts should form a colorless solution when dissolved in water?

(A) CuCl₂
(B) FeCl₃
(C) CrCl₃
(D) CaCl₂

47. Which of the following can act as a Lewis base?

(A) BF₃
(B) CH₄
(C) PH₃
(D) NH₄⁺

48. The shape of the phosphite ion, PO₃³¯, is best described as

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

49. What is the ground state electron configuration of the Mn²⁺ ion?

(A) [Ar] 4s¹ 3d⁵
(B) [Ar] 4s² 3d³
(C) [Ar] 3d⁵
(D) [Ar] 3d⁴

50. Which group of metals has the lowest melting points?

(A) alkali metals
(B) alkaline earth metals
(C) transition metals
(D) lanthanide metals

51. Which solution has the highest electrical conductivity?

(A) 0.05 M KCl
(B) 0.05 M HF
(C) 0.05 M NH₃
(D) 0.05 M CaCl₂

52. Which of the following compounds exhibit geometric isomerism?

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

53. What hybrid orbitals are used by each nitrogen atom in N₂F₂?

(A) sp
(B) sp²
(C) sp³
(D) p only

54. From the Lewis structures given below, choose the invalid one.

55. The compounds Br₂ (159.8 g mol¯¹) and ICl (162.4 g mol¯¹) have similar molar masses, yet ICl boils at 97 °C and Br₂ boils at 59 °C. Which statement best explains this difference?

(A) ICl has a longer bond that Br₂.
(B) ICl is ionic, while Br₂ is covalently bonded.
(C) ICl has a measurable dipole movement, but Br₂ does not.
(D) ICl has a stronger bond than Br₂.

56. Which one of the following salts is expected to give a red color in a flame test?

(A) KCl
(B) NaCl
(C) CaCl₂
(D) BaCl₂

57. Which of the following metals exhibits the greatest variety of oxidation states?

(A) Ba
(B) Y
(C) Cr
(D) Tl

58. In spontaneous beta particle (b¯) emission, what is the source of the emitted electron?

(A) the nucleus
(B) the 1s orbital
(C) the outermost occupied orbital
(D) a random orbital

59. Which one of the compounds below is classified as a peroxide?

(A) MgO
(B) Na₂O
(C) BaO₂
(D) KO₂

60. Which of the following sets of ions is generally associated with "hard" water?

(A) Na⁺, Ca²⁺
(B) Na⁺, K⁺
(C) Ca²⁺, Mg²⁺
(D) Mg²⁺, SO₄²¯

61. The boiling points of four liquids are shown. Which liquid has the highest vapor pressure at 25 °C?

(A) ethyl ether
(B) acetone
(C) ethanol
(D) water

62. Which term best describes a 0.10 M solution of Fe(NO₃)₃?

(A) saturated
(B) neutral
(C) acidic
(D) basic

63. Very strong acids, such as HNO₃ and HCl, appear to be equally strong in water. This "leveling effect" of water occurs because

(A) OH¯ is a stronger base than the conjugate bases of HNO₃ and HCl.
(B) H₃O⁺ is a stronger acid than HNO₃ and HCl.
(C) H₂O is a base than the conjugate bases of HNO₃ and HCl.
(D) H₂O is a weaker base than the conjugate bases of HNO₃ and HCl.

64. If the equilibrium P₄(g) + 6 Cl₂(g) <===> 4 PCl₃(g) is established by adding equal numbers of moles of P₄ and Cl₂ to an evacuated flask, which of the following must be true at equilibrium?

(A) [Cl₂] > [PCl₃]
(B) [Cl₂] < [P₄]
(C) [PCl₃] > [P₄]
(D) [P₄] = [PCl₃]

65. Consider the equilibrium NH₄HS(s) <===> NH₃(g) + H₂S(g). A 4.65 g sample of solid NH₄HS is placed in an evacuated 3.0 L flask at 35 °C and allowed to reach equilibrium at which time the total pressure in the flask is 0.82 atm. What is the value of K_p at this temperature?

(A) 0.67
(B) 0.41
(C) 0.17
(D) 0.45

66. When propanol, CH₃CH₂CH₂OH, is oxidized to propanoic acid, CH₃CH₂COOH, what is the number of electrons on the right side of the half-reaction?

(A) 2
(B) 3
(C) 4
(D) 5

67. What is the [Cu²⁺] in the cell Zn / Zn²⁺ (0.05 M) // Cu²⁺ (X M) / Cu if the cell voltage is 1.03 V?

(A) 0.12 M
(B) 0.0002 M
(C) 0.05 M
(D) 0.0035 M

68. Ten amperes are passed through molten aluminum chloride for 5.5 hours. How many grams of aluminum metal could be produced by this electrolysis?

(A) 18.5
(B) 55.4
(C) 91.2
(D) 273

69. What is the DG° for the reaction 2 Al³⁺ + 6 I¯---> 2 Al + 3 I₂, at 298 K?

(A) +1300 kJ
(B) -650 kJ
(C) -420 kJ
(D) 650 kJ

70. How many of the following would shift this reaction to the right?

PbO₂(s) + 4 H⁺ + Cu(s) ---> Pb²⁺+ 2 H₂O + Cu²⁺

Acid is added.
PbO₂ is added.
The [Pb²⁺] is increased.
The pH is increased.
Sulfide ion is added.

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