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U.S. National Chemistry Olympiad

1990 Local Section Test

Notes


1. DDT, a powerful pesticide, has 14 carbon atoms per molecule. The compound is 47% carbon by mass. What is the approximate molar mass of DDT?

(A) 80 g mol¯1
(B) 120 g mol¯1
(C) 180 g mol¯1
(D) 360 g mol¯1

2. Aluminum has a density of 2.7 g mL¯1. How much volume will be occupied by 4.0 mol of aluminum?

(A) 10.0 mL
(B) 20.0 mL
(C) 40.0 mL
(D) 80.0 mL

3. The atmosphere is approximately 71% N2 and 29% O2 by volume. What is the mass percentage of N2 in the atmosphere?

(A) <71%
(B) >71%
(C) 71%
(D) more data needed

4. A 50.0 g sample of an inert gas occupies 3.1 L at 25°C and 1.0 atm pressure. What gas might it be?

(A) He
(B) Ne
(C) Ar
(D) Kr

5. Given the following equation,

C3H3 + O2 ---> CO2 + H2O

what is the sum of the coefficients when the equation is balanced using smallest whole numbers?

(A) 9
(B) 13
(C) 15
(D) 25

6. A solute is dissolved in toluene, lowering the freezing point of the toluene. The amount of change in freezing point depends on

(A) the number of particles formed by the solute when dissolved.
(B) the molar mass of the solvent.
(C) the freezing point of the solute.
(D) the atmospheric pressure.

7. An ideal gas at 300 K and 1.00 atm is contained in a flexible vessel. Its volume is then reduced to one-half the original volume and the pressure remains unchanged. The new temperature is

(A) 75 K
(B) 150 K
(C) 300 K
(D) 600 K

8. How many moles of oxygen are contained in 14.8 g of magnesium nitrate (molar mass = 148 g mol¯1)?

(A) 0.100
(B) 0.200
(C) 0.300
(D) 0.600

9. At room temperature, in which of the following are the molecules farthest apart?

(A) water
(B) flourine
(C) bromine
(D) iodine

10. How many milliliters of 0.20 M AlCl3 solution would be necessary to precipitate all of the Ag+ from 45 mL of a 0.20 M AgNO3 solution?

AlCl3(aq) + 3 AgNO3(aq) ---> Al(NO3)3(aq) + 3 AgCl(s)

(A) 15 mL
(B) 30 mL
(C) 45 mL
(D) 60 mL

11. What is the pH of a 4.4 x 10¯3 M HBr solution?

(A) 1.44
(B) 2.36
(C) 3.64
(D) 5.43

12. The density of a typical gas is

(A) 1 g mL¯1
(B) 1 g L¯1
(C) 100 g L¯1
(D) 0.1 g L¯1

13. When titrating a weak acid with a strong base, which of the following indicators would be best to use?

(A) quinaldine red, pKa = 2.75
(B) bromeresol green, pKa = 7.10
(C) cresol purple, pKa = 8.32
(D) not enough information is provided

14. Three weak acids and their pKa values are listed below. Assuming equal molar initial concentration in water, which solution will have the highest pH?

  Acid pKa
(A) HF 3.17
(B) HN3 4.72
(C) HCN 9.32
(D) Insufficient data is given to make this prediction.

15. The net-ionic equation for the addition of acetic acid to a solution of potassium hydroxide is

(A) C2H3O2¯ + KOH ---> KC2H3O2 + OH¯
(B) C2H3O2¯ + H2O ---> HC2H3O2 + OH¯
(C) HC2H3O2 + OH¯ ---> C2H3O2¯ + H2O
(D) H+ + OH¯ ---> H2O

16. Which of the following atoms has the largest atomic radius?

(A) K
(B) Sc
(C) Fe
(D) Kr

17. For the reaction shown below, which statement is true?

2Fe + 3 CdCl2 <===> 2 FeCl3 + 3Cd

(A) Fe is the oxidizing agent
(B) Cd undergoes oxidation
(C) Cd is the reducing agent
(D) Fe undergoes oxidation

18. Which of the following refers to the ground-state electron configuration of an atom?

(A) 1s12s1
(B) [Kr]5p1
(C) [Ne]3s13p2
(D) [Ar]4s23d6

19. What is the potential in volts for the spontaneous reaction between the Ag/Ag2+ and Zn/Zn2+ half-cells?

Zn2+ + 2e¯ ---> Zn E° = -0.763V
Ag+ + 1e¯ ---> Ag E° = 0.799V
(A) -2.361
(B) -1.562
(C) 1.562
(D) 2.361

20. The maximum number of electrons in an atom that can have quantum numbers n = 2, l = 1 is

(A) 2
(B) 6
(C) 8
(D) 10

21. Moving down a particular periodic group of representative elements, which one of the following properties will increase?

(A) metallic character
(B) electronegativity
(C) ionization energy
(D) electron affinity

22. Which of the following pairs contains isoelectronic species?

(A) Be and Li+
(B) P+ and S¯
(C) N2¯ and Ne
(D) O2¯ and Na+

23. Which species is diamagnetic in the ground state?

(A) N
(B) Zn2+
(C) Cu2+
(D) O¯

24. Graphite and diamond are

(A) the same substance but two different elements.
(B) the same element, but two different substances.
(C) two different substances and two different elements.
(D) the same substance and the same element.

25. Which element can attain the highest oxidation number?

(A) F
(B) C
(C) Cl
(D) N

26. The half-life of 99Tc is 6.00 hours. If it takes exactly 12.00 hours for the manuacturer to deliver a 99Tc sample to a hospital, how much must be shipped in order for the hospital to receive 10.0 mg?

(A) 40.0 mg
(B) 30.0 mg
(C) 20.0 mg
(D) 15.0 mg

27. What are the products of the reaction below?

CaCO3(s) + heat -->

(A) CaC + O2
(B) CaO2 + CO
(C) CaO + CO
(D) CaO + CO2

28. Which of the following substances is a gas at room temperature?

(A) KBr
(B) C2H5OH
(C) NH4Cl
(D) Cl2

29. Which of the following compounds violates the octet rule?

(A) ICl4¯
(B) NO3¯
(C) BF4¯
(D) HCN

30. What is the best Lewis structure for HOBr, hypobromous acid?

31. Which of the following sets has the atoms and/or ions in the correct order of increasing size?

(A) Ne < F¯ < O2¯
(B) Br¯ < Cl¯ < F¯
(C) Na+ < Mg2+ < Al3+
(D) P < S < Cl

32. A mixture of hydrogen and nitrogen are allowed to come to equilibrium at a specified temperature according to the reaction.

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

Analysis of the same equilibrium mixture shows that it contains 1.5 atm NH3, 2.0 atm N2 and 3.0 atm H2. What is the value of the equilibrium constant, Kp?

(A) 0.042
(B) 0.16
(C) 0.25
(D) 6.5

33. The pH of an aqueous solution of an unknown compound is 8.22. This solution could contain a

(A) low concentration of a strong base.
(B) high concentration of a strong acid.
(C) high concentration of a strong base.
(D) low concentration of a strong acid.

34. Which of the following salts has the greatest molar solubility in pure water?

(A) CaCO3 (Ksp = 8.7 x 10¯9)
(B) CuS (Ksp = 8.5 x 10¯45)
(C) Ag2CO3 (Ksp = 6.2 x 10¯12)
(D) Pb9IO3)2 (Ksp = 2.6 x 10¯13)

35. Nitric acid 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?

(A) NH4+
(B) NO
(C) N2O5
(D) N2O4

36. An atom of iron-56, 56Fe, contains

(A) 26 electrons, 26 protons, 56 neutrons.
(B) 56 electrons, 26 protons, 26 neutrons.
(C) 56 electrons, 56 protons, 26 neutrons.
(D) 26 electrons, 26 protons, 30 neutrons.

37. The number of valence-shell bonding electrons to be distributed in the Lewis electron-dot model for HNNN is

(A) 6
(B) 10
(C) 11
(D) 16

38. What is the mass of metallic silver produced by a 1.50 amp current passed through a solution of silver nitrate for 20 minutes?

(A) 0.05 g
(B) 1.5 g
(C) 3.0 g
(D) 6.0 g

39. What is the heat capacity of copper if 15.0 g of copper warms from 20.0°C to 38.6°C when 108 J of heat are added to it?

(A) 0.387 J mol¯11
(B) 2.58 J mol¯11
(C) 11.85 J mol¯11
(D) 24.6 J mol¯11

40. If atom X has 3 valence electrons and atom Y has 6 valence electrons, what formula is expected for the simplest ionic compound composed of X and Y?

(A) XY2
(B) X2Y
(C) X2Y3
(D) X3y2

41. A sample of helium gas, present in a closed container of constant volume, is heated until the absolute temperature has been doubled. Which one of the following properties also doubles?

(A) destiny
(B) number of molecules
(C) average molecular speed
(D) pressure

42. What is the oxidation state of arsenic in H3AsO3?

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

43. Which of the following diatomic molecules has the strongest bonds?

(A) H2
(B) F2
(C) Cl2
(D) O2

44. What is the geometrical arrangement of atoms in the SO23¯ ion?

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

45. For the reaction system

2HgO(s) + heat <===> 2 Hg(l) + O2(g)

which conditions favor maximum conversion of the reactant to products?

(A) high temperature and high pressure
(B) high temperature and low pressure
(C) low temperature and low pressure
(D) low temperature and high pressure

46. Which of the following elements should react most readily with water?

(A) sulfur
(B) phosphorus
(C) silicon
(D) sodium

47. For which of the following equations is the change in enthalpy at 25°C and 1 atm equal to Df of CH2O(l)

(A) C(g) + H2(g) + (1/2) O2(g) ---> CH2O(l)
(B) C(s) + H2(g) + (1/2) O2(g) ---> CH2O(l)
(C) C(g) + 2 H2(g) + O(g) ---> CH2O(l)
(D) CO(g) + H2(g) ---> CH2O(l)

48. In pure water, Co(OH)2 is quite insoluble (Ksp = 2.5 x 10¯16). In acidic solution, the solubility is expected to

(A) increase.
(B) decrease.
(C) be the same as in basic solution.
(D) vary, but in an unpredictable manner.

49. Sulfur dioxide will react with oxygen in the presence of a catalyst to form sulfur trioxide.

2 SO2(g) + O2(g) <===> 2 SO3(g)

When 1.00 mol of O2 and 2.00 mol of SO2 are placed in a container and allowed to come to equilibrium at a certain temperature, the mixture is found to contain 1.75 mol of SO2. What is the amount of O2 at equililibrium?

(A) 0.00 mol
(B) 0.125 mol
(C) 0.250 mol
(D) 0.875 mol

50. Which noble gas has the highest boiling point?

(A) Ne
(B) Ar
(C) Kr
(D) Xe

51. A likely bond angle in a molecule with an sp3 hybridized central atom is

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

52. Which of the following pairs of atomic orbital on two adjacent atoms can form a pi-bond?

(A) s and px
(B) px and py
(C) px and px
(D) sp3 and pz

53. The rate law of a certain reaction is

rate = k [A] [B]2

The units of k, with time measured in seconds, is

(A) s¯1
(B) M¯12
(C) M¯21
(D) M¯1

54. The rate constant of a reaction is generally expected to

(A) be independent of temperature
(B) decrease with increasing temperature
(C) increase with increasing temperature
(D) increase with increasing temperature, only if the reaction is endothermic.

55. The rate law of the reaction

2 H2 + 2 NO ---> N2 + 2 H2O

is second order in [NO] and first order [H2]. If [NO] is doubled and [H2] is halved, the rate of the reaction will

(A) increase by a factor of 4
(B) remain the same
(C) increase by a factor of 6
(D) increase by a factor of 2

56. Cl2O is a yellowish-red gas at room temperature. The strongest intermolecular forces present in Cl2O are

(A) dipole-dipole forces.
(B) London forces.
(C) hydrogen bonds.
(D) covalent bonds.

57. A 0.10 g sample of zinc is added to excess hydrochloric acid. When the reaction is complete, what is the volume of gas, in mL, at STP that was evolved by the metal sample?

Zn(s) + 2 H+(aq) ---> Zn2+(aq) + H2(g)

(A) 10
(B) 17
(C) 34
(D) 68

58. The reaction below occurs in aqueous solution.

MnO4¯ + NO2 ---> MnO2 + NO3¯

When the reaction is balanced, what is the coefficient of H+?

(A) 2
(B) 4
(C) 6
(D) 8

59. A reaction has DH° = 34.3 kJ and DS = 15.6 J K¯1. Assuming DH° and DS° do not change appreciably with temperature, what is the minimum temperature above which the reaction will be spontaneous?

(A) 2.2 K
(B) 22 K
(C) 220 K
(D) 2200 K

60. What is the wavelength of light, in meters, that has a frequency of 1.5 x 10181?

(A) 2.0 x 10¯8
(B) 2.0 x 10¯10
(C) 5.0 x 109
(D) 5.0 x 1011

61. For which of the following molecules are resonance structures necessary to describe the bonding satisfactorily?

(A) O3
(B) CO2
(C) OF2
(D) PF3

62. Which of the following will NOT make a buffer? (Assume equal volumes of each solution.)

(A) 0.10 M NaF and 0.20 M HF
(B) 0.10 M HF and 0.05 M NaOH
(C) 0.10 M NaF and 0.03 M HCl
(D) 0.10 M NaF and 0.15 M HCl

63. If a weak acid, HA, is 1.0% dissociated in aqueous solution and the [A¯] = 2.0 x 10¯3 M, what is the Ka for the acid?

(A) 4.0 x 10¯6
(B) 2.0 x 10¯5
(C) 1.0 x 10¯2
(D) 5.0 x 104

64. Which of the following cations is colorless in aqueous solution?

(A) Cu2+
(B) Fe3+
(C) Zn2+
(D) Ni2+

65. The solubility product of CaF2 is 4.3 x 10¯11. What is the molar solubility of CaF2 in a 0.050 M solution of Ca(NO3)2?

(A) 2.2 x 10¯4 M
(B) 1.0 x 10¯5M
(C) 1.5 x 10¯5 M
(D) 8.4 x 10¯10 M

66. If 12.5 g of a protein are used to prepare 1.0 L of an aqueous solution having a osmotic pressure of 4.0 torr at 20°C, what is the molar mass of the protein?

(A) 219
(B) 5.7 x 103
(C) 5.7 x 104
(D) 2.6 x 107

67. A sample of hydrated copper (II) sulfate, CuSO4 . xH2O weighs 5.29 g. When the water is driven off, the anhydrous form weighs 3.38 g. What is the value of x in the formula of the hydrated salt?

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

68. If the free energy of formation, Df, of HI(g) is 1.7 kJ mol¯1, what is the equilibrium constant, K, at 25°C, for the reaction

HI(g) <===> 1/2 H2(g) + 1/2 I2(g)

(A) 0.25
(B) 0.50
(C) 2.0
(D) 4.0

69. Which of the following processes is accompanied by a decrease in entropy (DS < 0)?

(A) H2O(l) <==> H2O(g)
(B) NH4Cl(s) <==> HCl(g) + NH3(g)
(C) N2(g) + O2(g) <==> 2NO(g)
(D) 2 IBr(g) <==> I2(s) + Br2(l)

70. Which of the following, when in the solid state has a crystal structure containing discrete molecules?

(A) CO2
(B) SiO4
(C) MgO
(D) SiC