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U.S. National Chemistry Olympiad

1995 Local Test

Notes


1. Which gas could be collected over water?

(A) ammonia
(B) carbon dioxide
(C) hydrogen chloride
(D) oxygen

2. When sodium oxide, Na2O, is added to water, the major products expected are

(A) Na+ and OH¯ ions
(B) Na+ ions and H2O
(C) Na+ and O2¯ ions
(D) Na+ and OH¯ ions, and O2 gas

3. Which metal will react most vigorously with water?

(A) Na
(B) K
(C) Mg
(D) Ca

4. When 21.410 g of copper are added to a graduated cylinder containing 12.5 mL of water, the volume rises to 14.9 mL. What is the density of copper?

(A) 8.9208 g mL¯1
(B) 8.921 g mL¯1
(C) 8.92 g mL¯1
(D) 8.9 g mL¯1

5. Which of these gases is least abundant in the Earth's atmosphere?

(A) argon
(B) carbon monoxide
(C) nitrogen
(D) oxygen

6. The percentage of water in a hydrated salt is to be determined by weighing a sample of the hydrate, heating it to drive off the water, and weighing the anhydrous product. Which procedural error(s) could result in a percentage of water that is too low?

Procedural Mistakes
I. not heating the hydrate long enough
II. weighing the anhydrous sample while it is still warm
III. using a balance tht reads 0.12 g high for both weighings

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

7. How many grams of Ba(OH)2 . 8H20 (molar mass = 315.4 g mol¯1) are needed to prepare 250. mL of solution in which the [OH¯] = 0.180 M?

(A) 28.4
(B) 14.2
(C) 7.10
(D) 3.86

8. Ammonia is prepared by the Haber Process.

N2 + 3 H2 <===> 2 NH3

If 2 mol of N2 and 3 mol of H2 are combined, the amount of NH3 that would be formed if all of the limiting reactant were used up is known as the

(A) limited yield
(B) percent yield
(C) product yield
(D) theoretical yield

9. Calculate the percent by mass of hydrogen in diethylamine, (C2H5)2NH.

(A) 11.3%
(B) 13.7%
(C) 15.2%
(D) 23.6%

10. The insecticide Lindane®, has the emperical formula, CHCl. How many carbon atoms does each Lindane molecule contain?

Molar Mass of Lindane = 290.8 g mol¯1

(A) 2
(B) 3
(C) 4
(D) 6

11. 2.0 x 10¯3 mol of a chlorine containing compound, when placed in water, reacted completely with 40.0 cm3 of 0.1 M AgNO3. The compound could be

(A) NaCl
(B) MgCl2
(C) AlCl3
(D) TiCl4

12. Which reaction will produce a precipitate when 0.1 M aqueous solutions are mixed?

(A) NaOH + H2S --->
(B) CaCl2 + K2CO3 --->
(C) Al(NO3)3 + Na2SO4 --->
(D) CuSO4 + NH4Cl --->

13. What volume of oxygen gas (O2), is measured at 0 °C and 1 atm, is needed to completely burn 1 L of propane (C3H8) measured at the same onditions?

(A) 10 L
(B) 7 L
(C) 6 L
(D) 5 L

14. The number of moles of KOH in 500 mL of solution to be determined by titrating 10.00 mL portions of the KOH solution with 0.115 M HCl solution. If 18.72 mL of the HCl solution are needed to titrate each 10.00 mL portion of the KOH solution, how many moles of KOH are present in the original solution?

(A) 0.00215 mol
(B) 0.00430 mol
(C) 0.108 mol
(D) 0.215 mol

15. A certain element, Z, reacts with oxygen to form the compound Z2O5. If 0.364 g of the element form 0.552 g of the compound, what is the atomic molar mass of the element?

(A) 12.3 g mol¯1
(B) 74.6 g mol¯1
(C) 77.4 g mol¯1
(D) 153 g mol¯1

16. Which gas would be expected to have a density less than that of argon under the same conditions?

I. carbon dioxide
II. chlorine
III. oxygen

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

17. How will the vapor pressure and the boiling temperature of H2O be affected by the addition of NaCl?

  Vapor Pressure Boiling Temperature
(A) decreases increases
(B) decreases decreases
(C) increases decreases
(D) increases increases

18. When 1.2 g of sulfur is melted with 15.0 g of naphthalene, the solution freezes at 77.88 °C. What is the molar mass (in g mol¯1) of this form of sulfur?


Data for Naphthtalene
Melting point, m.p. 80.00 °C
Freezing point depression
constant, kf
-6.80 °C m¯1
(A) 82
(B) 118
(C) 256
(D) 560

19. An 81.4 mL sample of H2 is collected over water at 23.0 °C and 752.3 mmHg. How many moles of H2 are present?

Vapor pressure at 23.0 °C for H2O equals 21.1 mmHg

(A) 1.61 x 10¯3 mol
(B) 3.22 x 10¯3 mol
(C) 3.70 x 10¯3 mol
(D) 6.44 x 10¯3 mol

20. Equal moles of hydrogen and oxygen gas are placed in a container with a pinhole through which both can escape. What fraction of the oxygen escapes in the time required for one-half of the hydrogen to escape?

(A) 1/8
(B) 1/4
(C) 3/8
(D) 1/2

21. For which of these processes is the sign of the enthalpy change different from the others?

(A) Al2O3(s) ---> 2 Al(s) + 3/2 O2(g)
(B) H2O(s) ---> H2O(l)
(C) Cl2(g) ---> 2Cl(g)
(D) Cl(g) + e¯ ---> Cl¯(g)

22. The standard ethalpy of formation (DH°) for nitrogen(IV) oxide is the enthalpy change for the reaction

(A) N(g) + 2O(g) ---> NO2(g)
(B) 1/2 N2(g) + O2(g) ---> NO2(g)
(C) 1/2 N2O4(g) ---> NO2(g)
(D) NO(g) + 1/2 O2 ---> NO2(g)

23. In a bomb calorimeter, reactions are carried out at

(A) constant volume.
(B) constant pressure.
(C) 1 atm pressure and 25 °C.
(D) 1 atm pressure and 0 °C.

24. For the process O2(g) ---> 2 O(g), [delta]H° = +498 kJ.

What would be predicted for the sign of [delta]Srxn and the conditions under which this reaction would be spontaneous?

  [delta]Srxn Spontaneous
(A) positive at low temperatures only
(B) positive at high temperatures only
(C) negative at high temperatures only
(D) negative at low temperatures only

25. For the reaction

NH4Cl(s) --> NH3(g) + HCl(g)

[delta]H° = +176kJ and [delta]G° = +91.2 kJ at 298 K. What is the value of [delta]G at 1000 K?

(A) -109 kJ
(B) -64 kJ
(C) +64 kJ
(D) +109 kJ

26. Consider the reaction

Hg(l) + 2 Ag+(aq) ---> Hg2+(aq) + 2 Ag(s)

What is the enthalpy change for this reaction if [delta]H°f for Ag+ (aq) is +105.6 kJ mol¯1 and for Hg2+ (aq) is +171.1 kJ mol¯1?

(A) 65.5 kJ are evolved per mole of Hg.
(B) 65.5 lK are absorbed per mole of Hg.
(C) 40.1 kJ are evolved per mole of Hg.
(D) 40.1 kJ are absorbed per mole of Hg.

27. For the reaction

2 H2O2(aq) ---> 2 H2O(aq) + O2(g)

How does the value of [delta][H2O2] / [delta] t compare with that of [delta][O2] / [delta] t?

(A) [delta][H2O2] / [delta] t = [delta] [O2] / [delta] t
(B) - [delta][H2O2] / [delta] t = [delta] [O2] / [delta] t
(C) [delta] [H2O2] / [delta] t = 2[delta] [O2] / [delta] t
(D) - [delta][H2O2] / [delta] t = 2[delta] [O2] / [delta] t

28. Which is (are) constant over the course of a first order reaction?

I. reaction rate
II. rate constant

(A) I only
(B) II only
(C) Both I and II
(D) Neither I nor II

29. A wooden splint that glows in air burns when place in a test tube of oxygen beause the oxygen

(A) serves as a catalyst
(B) increases the temperature.
(C) is at a higher concentration in the test tube.
(D) has a higher order when it is pure.

30. For a first order reaction that has a half-life of 72 s at 80 °C, what is the value of the rate constant, k?

(A) 9.6 x 10¯31
(B) 6.2 x 10¯31
(C) 4.2 x 10¯31
(D) 1.4 x 10¯31

31. What is the activation energy for a reaction if its rate doubles when the temperature is raised from 20 °C to 35 °C?

(A) 342 kJ mol¯1
(B) 269 kJ mol¯1
(C) 34.7 kJ mol¯1
(D) 15.1 kJ mol¯1

32. Which of these changes has no effect on the equilibrium position of a given reaction?

(A) concentration
(B) temperature
(C) use of a catalyst
(D) partial pressure of a gaseous reactant

33. Consider this reaction

P4(s) + 6 Cl2(g) <===> 4 PCl3(l)

The correct Kc expression for this reaction is

(A) Kc = 1 / [Cl2]6
(B) Kc = [PCl3]4 / [P4][Cl2]6
(C) Kc = [PCl3]4 / [Cl2]6
(D) Kc = [P4][Cl2]6 / [PCl3]4

34. Consider this equilibrium system.

CaCO3(s) <===> CaO(s) + CO2(g)

If this system is at equilibrium, which of the following will occur if some solid CaCO3 is added to it?

I. The partial pressure of the CO2 will increase.
II. The mass of the solid CaO will decrease.
III. The system will remain unchanged.

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

35. The Ksp of CuCl is 1.9 x 10¯7 at 25 °C. What is the solubility of CuCl in mol L¯1?

(A) 3.6 x 10¯14
(B) 1.9 x 10¯7
(C) 4.4 x 10¯4
(D) 8.8 x 10¯4

36. At 0 °C the ion product constant of water, Kw, is 1.2 x 10¯15. The pH of pure water at this temperature is

(A) 6.88
(B) 7.00
(C) 7.46
(D) 7.56

37. What is the [H+] in a 0.010 M solution of HCN?

The equilibrium constant, Ka, for HCN equals 6.2 x 10¯10

(A) 3.6 x 10¯3 M
(B) 2.5 x 10¯6 M
(C) 1.0 x 10¯7 M
(D) 6.2 x 10¯10 M

38. HCN (aq) + HCO3¯ (aq) <===> CN¯ (aq) + H2CO3 (aq)

If the value of the equilibrium constant, K, is less than 1, what is the strongest base in this system?

(A) HCN
(B) HCO3¯
(C) CN¯
(D) H2CO3

39. A 0.100 M solution of acetic acid (Ka = 1.8 x 10¯5) is titrated with a 0.1000 M solution of NaOH. What is the pH when 50% of the acid has been neutralized?

(A) 2.38
(B) 4.74
(C) 5.70
(D) 7.00

40. The conjugate acid of the bicarbonate ion, HCO3¯, in H2O is

(A) H3O+
(B) CO32¯
(C) OH¯
(D) H2CO3

41. The sodium salt, NaA, of a weak acid is dissolved in water and no other substance is added. Which of the following statements is corrected?

(A) [H+] = [A¯]
(B) [H+] = [OH¯]
(C) [A¯] = [OH¯]
(D) [HA] = [OH¯]

42. Which of these ions is predicted to produce the most acidic solution when dissolved in H2O?

(A) K+
(B) Ba2+
(C) Co2+
(D) Fe3+

43. When 0.10 M solutions of the solutes; HClO4, NH4Br, KOH, KCN, are arranged in order in increasing [H+], the correct order is

(A) KOH < KCN < NH4Br < HClO4
(B) KCN < KOH < HClO4 < NH4Br
(C) HClO4 < NH4Br < KCN < KOH
(D) NH4Br < HClO4 < KOH < KCN

44. Which indicator is most appropriate for titrating a 0.100 M solution of NH3 with 0.100 M HNO3?

The equilibrium constant, Ka, for NH3 equals 1.8 x 10¯5

  Indicator pH color transition range
(A) cresol red 1-2
(B) methyl red 4-6
(C) phenolphthalein 8-10
(D) alizarin yellow 10-12

45. Based on everyday chemical knowledge, which metal is the most easily oxidized?

(A) Fe
(B) Pb
(C) Ag
(D) Na

46. In which pair does the named substance have the same oxidation number?

(A) sulfur in H2S2O7 and H2SO4
(B) mercury in Hg2+ and Hg22+
(C) oxygen in Na2O2 and H2O
(D) cobalt in Co(NH3)63+ and Co(NO3)2

47. The electrochemical series uses this reaction as a standard.

2 H+ + 2 e¯ --> H2

In order for this half cell to have a potential of 0.00 V, all of the following are required except

(A) a solution with [H+] concentration = 1.0 M
(B) hydrogen gas at a pressure of 1.0 atm.
(C) an external source of electrons
(D) an electrode made of an inert metal such as platinum.

48. What mass of platinum could be plated on an electrode from the electrolysis of a Pt(NO3)2 solution with a current of 0.500 A for 55.0 s?

(A) 27.8 mg
(B) 55.6 mg
(C) 45.5 mg
(D) 91.0 mg

49. Which principal quantum number transition in a hydrogen atom will release the greatest amount of energy?

(A) 1 ---> 3
(B) 3 ---> 5
(C) 4 ---> 2
(D) 6 ---> 4

50. Which element in its ground state has the greatest number of unpaired electrons?

(A) V (Z = 23)
(B) Fe (Z = 26)
(C) In (Z = 49)
(D) As (Z = 33)

51. For which element are the oxidation states given actually observed and the boldface state the most stable?

(A) Ca (Z = 20) +1, +2
(B) Cr (Z = 24) +2, +3, +6
(C) In (Z = 49) +3, +5
(D) Pb (Z = 82) +2, +4

52. Which property decreases from left to right across the periodic table?

(A) electron affinity
(B) electrical conductivity
(C) ionization energy
(D) maximum oxidation state

53. When the elements; S, Se and Cl are arranged in order of increasing atomic radius, the correct order is

(A) Se < S < Cl
(B) S < Cl < Se
(C) Cl < S < Se
(D) S < Se < Cl

54. Which bond is most polar?

(A) F-F
(B) F-O
(C) F-Cl
(D) F-S

55. Which molecule is polar?

(A) SO2
(B) CO2
(C) XeF2
(D) PCl5

56. Which electron-dot structure is most suitable for the thiocyanate ion, SCN¯?

57. When the carbon-oxygen bonds in the species below are arranged in order of increasing length (shortest first), what is the correct order?

I. CH3OH
II. CO2
III. CH3CO2¯

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

58. Which species can be described as having sp3d hybridization?

I. PCl5
II. SF4
III. I3¯

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

59. Which substance is expected to have the lowest boiling point?

(A) Si
(B) SiC
(C) SiO2
(D) Si4H10

60. How many sigma bonds and how many pi bonds are represented in this structure?


  sigma bonds pi bonds
(A) 6 3
(B) 7 2
(C) 8 1
(D) 9 0