U.S. National Chemistry Olympiad: 1995 Local Test

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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?

(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]S_rxn 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¯³ s¯¹
(B) 6.2 x 10¯³ s¯¹
(C) 4.2 x 10¯³ s¯¹
(D) 1.4 x 10¯³ s¯¹

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 K_c expression for this reaction is

(A) K_c = 1 / [Cl₂]⁶
(B) K_c = [PCl₃]⁴ / [P₄][Cl₂]⁶
(C) K_c = [PCl₃]⁴ / [Cl₂]⁶
(D) K_c = [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 K_sp 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, K_w, 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, K_a, 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 (K_a = 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, K_a, 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