U.S. National Chemistry Olympiad: 1991 Local
Go to answers
1. What is the maximum amount of MgCl2 that can be prepared from the
reaction of 10.0g of HCl with 10.0 of Mg(OH)2 ?
2 HCl + Mg(OH)2 ---> MgCl2 + 2 H2O
(A) 9.26 g
(B) 13.1 g
(C) 16.3 g
(D) 17.5 g
2. Phenylflouroform contains 57.54% C, 3.45% H, and 39.01% F. Its molecular formula is
the same as its simplest formula. How many carbon atoms are in one molecule of
3. What is the most likely formula for a compound of gallium and sulfur ?
4. A mixture of 2.30 x 1023 molecules N2 and 4.35 x 1023
molecules N2O has a total pressure of 2.00 atm. What is the partial pressure of
(A) 0.346 atm
(B) 0.654 atm
(C) 0.692 atm
(D) 1.11 atm
5. Which element should have the smallest first ionization energy ?
6. An aqueous solution containing 1.00 mol H2SO4 is mixed with an
aqueous solution containing 1.00 mol NaOH. The mixture is then evaporated to dryness. What
solid remains after evaporation ?
7. Which atom is the smallest ?
8. At room temperature and 1 atm pressure the molecules are farthest apart in
9. What is the net-ionic equation for the reaction of aqueous solutions of BaCl2
(A) Ba2+(aq) + 2Cl¯(aq) + 2K+(aq) + SO42¯(aq)
---> BaSO4(s) + 2K+(aq) + 2Cl¯(aq)
(B) Ba2+(aq) + SO42¯(aq) ---> BaSO4(s)
(C) K+(aq) + Cl¯(aq) ---> KCl(s)
(D) Ba2+(aq) + 2Cl¯(aq) + 2K+(aq) + SO42¯(aq)
---> BaSO4(s) + 2KCl(s)
10. When 80.00 mL of 0.200 M HNO3 is added to 120.00 mL of 0.150 M KOH, the
HNO3(aq) + KOH(aq) ---> KNO3(aq) + H2O(l)
occurs and the resulting solution is
(A) 0.0160 M KNO3
(B) 0.0180 M KNO3
(C) 0.0800 M KNO3 and 0.0100 M KOH
(D) 0.0160 M KNO3 and 0.0200 M KOH
11. Methanol burns in oxygen according to the equation
CH3OH + O2 ---> CO2 + H2O
What volume of oxygen is required to burn 5.0 L of gaseous methonal measured at the
same temperature and pressure ?
(A) 15.0 L
(B) 3.3 L
(C) 7.5 L
(D) 5.0 L
12. A 50 g sample of an ideal gas occupies a volume of 3.2 L at 40 °C and exerts a
pressure of 2.6 atm. What is its molar mass ?
(A) 150 g mol¯1
(B) 2.0 g mol¯1
(C) 77 g mol¯1
(D) 310 g mol¯1
13. When sodium metal reacts with an excess of water the products are
(A) Na2O(s) and H2(g)
(B) Na+(aq), OH¯(aq) and H2(g)
(C) NaH(s) and O2(g)
(D) Na2O2(s) and H2(g)
14. Which is least important in determining the physical behavior of a gas?
(D) chemical composition
15. Which pair of atoms should form the most polar bond ?
(A) B and F
(B) C and O
(C) O and F
(D) F and N
16. Which pair of ions should form the strongest ionic bond?
(A) Li+ and I¯
(B) Na+ and F¯
(C) Cs+ and F¯
(D) Na+ and O2¯
17. The kinetic-molecular theory predicts that two gases at the same temperature will
have the same
(A) average speed
(B) average kinetic energy
(D) rate of effusion
18. Which factors do not affect the vapor pressure of a liquid at equilibrium?
I. Intermolecular forces of attraction.
II. The volume of liquid present.
III. The temperature of the liquid
(A) I only
(B) II only
(C) I and II only
(D) II and III only
19. Which has the highest vapor pressure at room temperature ?
(A) a vegetable oil
(C) table sugar, C12H22O11
(D) table salt, NaCl
20. Identify the best Lewis structure for hydrogen cyanide.
21. Which oxide of chlorine exhibits the maximum oxidation state that really exists for
22. Which has the largest radius ?
23. What is the molarity of HCl if 450 mL of 2.3 M HCl is diluted to 1250 mL ?
(A) 0.36 M
(B) 0.50 M
(C) 0.83 M
(D) 1.8 M
24. The first five ionization energies in kJ mol¯1, for a particular
element are shown.
The element is likely to form a ionic compounds in which its charge is
25. Which compound, when dissolved in water, conducts electricity and forms a basic
26. Which pair constitutes a buffer ?
(A) HCl and KCl
(B) NaOH and NaCl
(C) HNO2 and NaNO2
(D) HNO3 and NH4NO3
27. Which oxide dissolves in water to give the strongest acid ?
28. Which is the strongest Bronsted base ?
29. In the reaction F¯ + H2O ---> HF + OH¯ an acid-base conjugate pair
(A) F¯ and H2O
(B) F¯ and OH¯
(C) H2O and HF
(D) F¯ and HF
30. Which is the anhydride of nitric acid ?
31. Which forms a basic aqeous solution ?
32. The best acid for preparing a buffer of pH = 3.2 has a Ka near
(A) 2.0 x 10¯5
(B) 6.0 x 10¯4
(C) 3.2 x 10¯2
(D) 4.0 x 10¯6
33. What is the pH of a 2.0 x 10¯3 M HCl solution ?
34. Consider a solution that contains only one ion, Ag+, Pb2+, or
Hg22+. The solution gave a precipitate with HCl that was insoluble
in hot water but was soluble in dilute ammonia solution. Which ion was present ?
(D) The information is insufficient to identify the ion
35. What is the partial pressure of CO2(g) if Kp for the reaction
is 1.04 atm ?
CaCO3(s) <===> CaO(s) + CO2(g)
(A) 0.52 atm
(B) 1.04 atm
(C) 1.08 atm
(D) 10.4 atm
36. The Ksp values for CaSO4,BaSO4, and Ag2SO4
are 2.0 x 10¯4, 1.5 x 10¯9, and 1.5 x 10¯5
respectively. If 0.010 M Na2SO4 is slowly added to a solution that
contains Ca2+, Ba2+, and Ag+ (each 0.10M), which solid
will precipitate last?
(D) More information is required
37. The Ksp of AgCl is 1.8 x 10¯10. If 10.00 mL of 0.0010 M NaCl
is combined with 20.00 mL of 0.0050 M AgNO3
(A) no reaction occurs
(B) a precipitate forms
(C) an initial precipitate forms which dissolves upon stirring.
(D) the silver ion concentration remainsas 0.005 M.
38. N2(g) + 3H2(g) <===> 2NH3(g) + 92.2 kJ
The partial pressure of N2 is not increased by
(A) decreasing container size
(B) adding NH3
(C) increasing temperature
(D) removing H2
39. Which value for an equilibrium constant, K, could indicate that the reaction
A(g) + B(g) ---> C(g)
goes nearly to completion?
40. Which is most soluble in a polar solvent?
41. When a liquid is placed in a closed container
(A) evaporation stops
(B) evaporation continues for a time then stops
(C) the vapor pressure above the liquid becomes one atmosphere
(D) evaporation and condensation continue to occur, but at equal rates
42. The sublimination of dry ice is an endothermic process. The energy absorbed is
required to overcome
(A) covalent bonds
(B) polar covalent bonds
(C) dipole forces
(D) dispersion forces
43. Compared to a 1.0 M aqueous solution of a nonelectrolyte, a 1.0 M solution of an
electrolyte will have a
(A) higher melting point and higher boiling point
(B) higher melting point and lower boiling point
(C) lower melting point and lower boiling point
(D) lower melting point and higher boiling point
44. Which is planar?
45. Calculate DH°f for
4 NH3(g) + 3 O2(g) ---> 2 N2(g) + 6
Given that DH°f for NH3(g) and H2O(g)
are -46.0 and -242 kJ mol¯1 respectively.
(A) -1268 kJ
(B) -196 kJ
(C) +1268 kJ
(D) +196 kJ
46. A white, odorless, crystalline solid melts after about ten seconds in a burner
flame. It is soluble in water and insoluble in carbon tetrachloride. Neither the melt nor
its aqueous solution conducts electricity. We conclude that the substance is
(A) an ionic compound
(B) a non-polar covalent compound
(C) a polar covalent compound
(D) a pure element
47. Given the standard reduction potentials,
|Cr3+ + 3e¯ ---> Cr
|Pb2+ + 2e¯ ---> Pb
what is the standard potential, E°, for the following reaction?
2 Cr + 3 Pb2+ ---> 2 Cr3+ + 3 Pb
(A) 0.61 V
(B) 0.87 V
(C) 1.09 V
(D) 1.87 V
48. A mixture of 0.10 mol H2 and 0.050 mol O2 are placed in a
bomb calorimeter with a heat capacity of 5.1 x 104 J °C¯1. The
initial temperature is 25.00 °C and the temperature after combustion is 25.56 °C. What
is the DH°f for H2O(l)? (Assume 100%
conversion to water.)
(A) -286 kJ mol¯1
(B) +190 kJ mol¯1
(C) -190 kJ mol¯1
(D) +286 kJ mol¯1
49. In the reaction
2MnO4¯ + 16H+ + 10Br¯ ---> 2Mn2+ +
8H2O + 5Br2
Br¯ ion is the
(A) oxidizing agent and is oxidized
(B) oxidizing agent and is reduced
(C) reducing agent and is oxidized
(D) reducing agent and is reduced
50. For BrF3, the orbital geometry of the central atom and the molecular
geometry are, respectively,
(A) trigonal planar and trigonal planar
(B) trigonal planar and trigonal bipyramidal
(C) trigonal bipyramidal and trigonal planar
(D) trigonal bipyramidal and T-shaped
51. How many electrons are needed to convert one nitrate ion into one ammonia molecule?
52. Which species below violates the octet rule?
53. What is the coefficient for H2O when the redox equation
Cu + NO3¯ + H+ ---> Cu2+ + NO + H2O
is balanced using smallest whole - number coefficients?
54. Which change will produce an increase in the entropy of a system?
(A) decreasing the temperature
(B) forming a precipitate in a solution
(C) decreasing the volume
(D) forming a gas from a solid reaction
55. Which will cause the value of the equilibrium constant for this reaction to
2A(g) + B(g) ---> C(g); DH° = 280 kJ
(A) adding a catalyst
(B) increasing the concentration of A
(C) increasing the concentration of C
(D) increasing the temperature
2CO(g) + O2(g) ---> 2CO2(g); DG° = -516 kJ
4MnO(s) + O2(g) ---> 2Mn2O3(s); DG° = -312 kJ
Calculate the value of DG° for
2MnO(s) + CO2(g) ---> Mn2O3(s) +
(A) +102 kJ
(B) -102 kJ
(C) +204 kJ
(D) -204 kJ
Br2(l) S° = 152.2 J mol¯1 K¯1 and DH°f = 0 kJ mol¯1
Br2(g) S° = 245.4 J mol¯1 K¯1 and DH°f
= 30.9 kJ mol¯1
Calculate the boiling point of liquid bromine.
(A) 58.4 K
(B) 332 K
(C) 125 K
(D) -147 K
58. Which is not expected to have a value of zero?
(A) DH°f for Cl2(g)
(B) DG°f for Na(s)
(C) S° for Ca(s)
(D) DH°f for Al(s)
59. The half-life of 14C is 5570 years. How many years will it take for 90%
of a sample to decompose?
(A) 5,570 years
(B) 17,700 years
(C) 18,600 years
(D) 50,100 years
60. A solution containg 2.10 g of Fe(NH4)2(SO4)2
. 6 H2O (M = 392.1 g mol¯1) was titrated with
acidic Na2Cr2O7 solution, requiring 41.60 mL of Na2Cr2O7
solution. What is the molarity of Na2Cr2O7?
6 Fe2+ + Cr2O72¯ + 14 H+
---> 6 Fe3+ + 2 Cr3+ + 7 H2O
(A) 0.0215 M
(B) 0.0387 M
(C) 0.0644 M
(D) 0.0744 M
61. What is the approximate H-O-H bond angle in H3O+?
62. What hybrid orbitals are used by nitrogen in N2F2?
(D) p only
63. The strongest interaction between atoms listed below is
(A) an instantaneous dipole-dipole interaction
(B) a dipole-dipole interaction
(C) a hydrogen bond
(D) a covalent bond
64. When a 5.0 g sample of an unknown compound is dissolved in 250.0 g of benzene, the
freezing point of benzene decreased 1.4 °C. The freezing point depression constant, Kf,
for benzene is 5.12 °C m¯1. What is the approximate molar mass of the
(A) 290 g mol¯1
(B) 73 g mol¯1
(C) 36 g mol¯1
(D) 140 g mol¯1
||Standard reduction potential, E°
|Cu2+ (aq) + 2e¯ ---> Cu(s)
|Ni2+ (aq) + 2e¯ ---> Ni(s)
When two half-cells are connected using a salt bridge,
(A) a galvanic cell will result in which Cu is the cathode
(B) a galvanic cell will result in which Cu is the anode
(C) an electrolytic cell will result in which Ni is the anode
(D) an electrolytic cell will result in which Ni is the anode
66. What is the maximum number of electrons that can occupy the 5d subshell?
67. For the isoelectronic series below, which species requires the least energy to
remove an outer electron?
68. In aqueous solution, iodide ion (in basic solution) is oxidized by hypochlorite
OCl¯(aq) + I¯(aq) ---> OI¯(aq) + Cl¯(aq)
The rate of formation of hypoiodite, OI¯, is given by the rate law
rate = ( k [OCl¯] [I¯] ) / [OH¯]
What is the overall reaction order for the formation of OI¯?
69. The dissociation of HI molecules, as shown below, occurs at a temperature of 629 K.
The rate constant, k = 3.02 x 10¯5 M¯1 s¯1.
2 HI(g) ---> H2(g) + I2(g)
What is the reaction order?
70. A sodium vapor street lamp emits yellow light with a wavelength of 589 nm. What is
the energy of this light per mole of protons?
(A) 3.37 x 10¯16 kJ mol¯1
(B) 2.35 x 10¯13 kJ mol¯1
(C) 0.201 kJ mol¯1
(D) 203 kJ mol¯1