U.S. National Chemistry Olympiad: 1991 Local Section Test

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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 Phenyflouroform ?

(A) 4
(B) 5
(C) 6
(D) 7

3. What is the most likely formula for a compound of gallium and sulfur ?

(A) GaS
(B) GaS₂
(C) Ga₂S₃
(D) Ga₃S₂

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 N₂ ?

(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 ?

(A) Ca
(B) Rb
(C) Al
(D) Cl

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 ?

(A) H₂SO₄
(B) NaOH
(C) NaHSO₄
(D) Na₂SO₄

7. Which atom is the smallest ?

(A) Rb
(B) Ag
(C) Sb
(D) I

8. At room temperature and 1 atm pressure the molecules are farthest apart in

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

9. What is the net-ionic equation for the reaction of aqueous solutions of BaCl₂ and K₂SO₄?

(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 reaction

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 (unbalanced)

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?

(A) volume
(B) pressure
(C) temperature
(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
(C) pressure
(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
(B) gasoline
(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 chlorine?

(A) ClO
(B) ClO₄
(C) Cl₂O₅
(D) Cl₂O₇

22. Which has the largest radius ?

(A) Na
(B) Na⁺
(C) F¯
(D) Ne

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

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

25. Which compound, when dissolved in water, conducts electricity and forms a basic solution ?

(A) HCl
(B) CH₃COOH
(C) CH₃OH
(D) KOH

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 ?

(A) SO₃
(B) CaO
(C) P₄O₁₀
(D) CO₂

28. Which is the strongest Bronsted base ?

(A) NH₂¯
(B) Cl¯
(C) PO₄³¯
(D) NO₂¯

29. In the reaction F¯ + H₂O ---> HF + OH¯ an acid-base conjugate pair is

(A) F¯ and H₂O
(B) F¯ and OH¯
(C) H₂O and HF
(D) F¯ and HF

30. Which is the anhydride of nitric acid ?

(A) NO
(B) NO₂
(C) N₂O₃
(D) N₂O₅

31. Which forms a basic aqeous solution ?

(A) NaI
(B) KNO₃
(C) BaCl₂
(D) Na₂CO₃

32. The best acid for preparing a buffer of pH = 3.2 has a K_a 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 ?

(A) 2.40
(B) 2.70
(C) 3.00
(D) 11.30

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 ?

(A) Pb²⁺
(B) Ag⁺
(C) Hg₂²⁺
(D) The information is insufficient to identify the ion

35. What is the partial pressure of CO₂(g) if K_p 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 K_sp 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?

(A) CaSO₄
(B) Ag₂SO₄
(C) BaSO₄
(D) More information is required

37. The K_sp 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?

(A) 10¹⁴
(B) 10¯¹⁴
(C) 1
(D) 0

40. Which is most soluble in a polar solvent?

(A) CF₄
(B) Br₂
(C) HCl
(D) BCl₃

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?

(A) H₃O⁺
(B) PCl₃
(C) CO₃²¯
(D) NH₄⁺

45. Calculate DH°_f for

4 NH₃(g) + 3 O₂(g) ---> 2 N₂(g) + 6 H₂O(g)

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,

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?

(A) 3
(B) 5
(C) 6
(D) 8

52. Which species below violates the octet rule?

(A) CF₄
(B) BF₃
(C) N₂O
(D) NO₃¯

53. What is the coefficient for H₂O when the redox equation

Cu + NO₃¯ + H⁺ ---> Cu²⁺ + NO + H₂O

is balanced using smallest whole - number coefficients?

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

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 increase?

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

56. Given

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) + CO(g)

(A) +102 kJ
(B) -102 kJ
(C) +204 kJ
(D) -204 kJ

57. Given

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⁺?

(A) 60
(B) 90
(C) 109
(D) 120

62. What hybrid orbitals are used by nitrogen in N₂F₂?

(A) sp
(B) sp²
(C) sp³
(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, K_f, for benzene is 5.12 °C m¯¹. What is the approximate molar mass of the compound?

(A) 290 g mol¯¹
(B) 73 g mol¯¹
(C) 36 g mol¯¹
(D) 140 g mol¯¹

65.

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?

(A) 2
(B) 6
(C) 10
(D) 14

67. For the isoelectronic series below, which species requires the least energy to remove an outer electron?

(A) K⁺
(B) Ar
(C) Cl¯
(D) S²¯

68. In aqueous solution, iodide ion (in basic solution) is oxidized by hypochlorite ion:

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¯?

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

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?

(A) 0
(B) 1
(C) 2
(D) 3

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