U.S. National Chemistry Olympiad: 1998 National Test

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1. Which pair of substances could be separated by mixing with water and filtering?

(A) NaNO₃ and K₂SO₄
(B) CH₃OH and CH₃COCH₃
(C) MgCO₃ and Fe(OH)₃
(D) KCl and CuS

2. Which gas produces an acidic solution when bubbled into water?

(A) NH₃
(B) Cl₂
(C) H₂
(D) CH₄

3. A dilute HCl solution is to be prepared from a more concentrated solution. Which of pieces of glassware will give the highest level of precision?

(A) graduated cylinder and volumetric flask
(B) two graduated cylinders
(C) pipet and graduated cylinder
(D) pipet and volumetric flask

4. Which substance is likely to show the greatest change in mass when exposed to air?

(A) Al
(B) NaOH
(C) CaCO₃
(D) SiO₂

5. Which is a yellow solid?

(A) C
(B) Na
(C) S₈
(D) I₂

6. An alloy of gold and silver contains 38.5% silver by mass and has a density of 14.6 g mL¯¹. What is the molar concentration of silver in this alloy?

(A) 52.1 mol L¯¹
(B) 45.6 mol L¯¹
(C) 3.57 mol L¯¹
(D) 2.64 mol L¯¹

7. As₂S₃ reacts with O₂ to give SO₂ and As₂O₃. What is the smallest whole number coefficient for O₂ when the equation for this reaction is balanced correctly?

_As₂S₃(s) + _O₂(g) ---> _As₂O₃(s) + _SO₂(g)

(A) 3
(B) 5
(C) 7
(D) 9

8. A 0.242 g sample of potassium is heated in oxygen. The result is 0.440 g of a crystalline compound. What is the formula of this compound?

(A) KO
(B) K₂O
(C) KO₂
(D) KO₃

9. What volume of 3.0 M Na₂SO₄ must be added to 25 mL of 1.0 M BaCl₂ to produce 5.0 g of BaSO₄?

(A) 7.2 mL
(B) 8.3 mL
(C) 10.0 mL
(D) 14 mL

10. These three equations describe an oxidation-reduction method for determining dissolved oxygen in water. How many moles of S₂O₃²¯ are equivalent of each mole of O₂?

1) 2Mn²⁺(aq) + 4OH¯(aq) + O₂(g) ---> 2MnO₂(s) + 2H₂O(l)

2) MnO₂(s) + 2I¯(aq) + 4H⁺(aq) ---> Mn²⁺(aq) + I₂(aq) + 2H₂O(l)

3) 2S₂O₃²¯(aq) + I₂(aq) ---> S₄O₆²¯(aq) + 2I¯(aq)

(A) 0.5
(B) 1
(C) 2
(D) 4

11. A 15 mL sample of 0.20 M MgCl₂ is added to 45 mL of 0.40 M AlCl₃. What is the molarity of Cl¯ ions in the final solution?

(A) 1.0 M
(B) 0.60 M
(C) 0.35
(D) 0.30 M

12. How many electrons are present in 2.0 x 10¯³ moles of ¹⁸₈O²¯?

(A) 1.2 x 10²¹
(B) 9.6 x 10²¹
(C) 1.2 x 10²²
(D) 1.9 x 10²²

13. In which change of state are covalent bonds broken?

(A) evaporation of water
(B) fusion of sodium chloride
(C) vaporization of diamond
(D) sublimation of dry ice

14. By what factor does the average velocity of a gaseous molecule increase when the absolute temperature is doubled?

(A) 1.4
(B) 2.0
(C) 2.8
(D) 4.0

15. Which species is expected to have the highest boiling point at 1 am pressure?

(A) O₂
(B) CO
(C) CO₂
(D) HCHO

16. What is the vapor pressure of a solution with a benzene to octane molar ratio of 2:1?

(A) 120 mmHg
(B) 320 mmHg
(C) 400 mmHg
(D) 680 mmHg

17. Which halogen in its standard state has the greatest absolute entropy per mole?

(A) F₂(g)
(B) Cl₂(g)
(C) Br₂(l)
(D) I₂(s)

18. Which combination of vapor pressure, intermolecular forces and DH_vaporization is matched correctly?

:	vapor pressure	intermolecular forces	DHvaporization
(A)	high	weak	small
(B)	high	strong	large
(C)	low	weak	large
(D)	low	strong	small

19. Calculate DH° for this reaction.

C₂H₂(g) + 2H₂(g) ---> C₂H₆(g)

(A) -312 kJ
(B) -26 kJ
(C) +26 kJ
(D) +312 kJ

20. For which process would DS° be expected to have the greatest positive value?

(A) O₂(g) + 2H₂(g) ---> 2H₂O(g)
(B) H₂O(s) ---> H₂O(l)
(C) H₂(g) + I₂(g) ---> 2HI(g)
(D) N₂O₄(g) ---> 2NO₂(g)

21. (1/2) N₂(g)+ (3/2) H₂(g) ---> NH₃(g)

For this reaction at 25 °C, DG°_f is -16.5 kJ mol¯¹. What is the equilibrium constant, K_eq, for this reaction at this temperature?

(A) 1.08
(B) 7.80 x 10²
(C) 4.57 x 10⁶
(D) 2.98 x 10³⁴

22. What are the signs for DH, DS, and DG for the freezing of liquid water at -10 °C?

23. Which of these factors affect the value of the specific rate constant for the reaction 2A(g) ---> B(g)?

I concentration
II pressure
III temperature

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

24. Which statement about work and heat is true?

(A) A given quantity of heat can be completely converted to work.
(B) Heat and work are both state functions.
(C) Some heat is always produced when work is done.
(D) Work is a state function, whereas heat is not.

25. The reaction between KMnO₄ and H₂C₂O₄ can be followed by monitoring the disappearance of the purple color of the MnO₄¯ ion. These data were obtained for the reaction carried out at a constant temperature of 25 °C.

Specify the order of this reaction with respect to [MnO₄¯] and [H₂C₂O₄]

	[MnO4¯]	[H2C2O4]
(A)	1st order	2nd order
(B)	2nd order	1st order
(C)	1st order	1st order
(D)	2nd order	2nd order

26. For which reaction order is the half-life independent of the initial concentration?

I first order
II second order

(A) I only
(B) II only
(C) both I and II
(D) neither I nor II

27. What is the activation energy for the reverse of this reaction?

N₂O₄(g) ---> 2NO₂(g)

Data for the given reaction is: DH = + 54.0 kJ and E_a = +57.2 kJ.

(A) -54.0 kJ
(B) +3.2 kJ
(C) +60.2 kJ
(D) +111.2 kJ

28. How does an increase in temperature affect the rates of the foward and reverse reactions for an exothermic reaction?

	rate of foward reaction	rate of reverse reaction
(A)	increases	increases
(B)	increases	decreases
(C)	decreases	increases
(D)	increases	remains the same

29. The reaction between chloroform, CHCl₃(g), and chlorine, Cl₂(g), to form CCl₄(g) and HCl(g) is believed to occur by this series of steps.

Step 1 Cl₂ ---> Cl(g) + Cl(g)
Step 2 CHCl₃(g) + Cl(g) ---> CCl₃(g) + HCl(g)
Step 3 CCl₃(g) + Cl(g) ---> CCl₄(g)

If this reaction is first order in CHCl₃ and half order in Cl₂, which statement about the relative rates of step 1, 2, and 3 is correct?

(A) Step 1 is the slowest.
(B) Steps 1 and 2 must both be slow.
(C) Step 2 must be slower than step 1.
(D) Step 3 must be the slowest

30. This reaction occurs readily above 500 °C.
2HgO(s) ---> 2Hg(l) + O₂(g)

What is the equilibrium for this reaction?

(A) K = [Hg]²[O₂] / [HgO]²
(B) K = [Hg][O₂] / [HgO]
(C) K = [Hg]²[O₂]
(D) K = [O₂]

31. Consider this equilibrium, for which DH < 0.

HgO(s) + 4I¯(aq) + H₂O(l) <===> HgI₄²¯ + 2OH¯

Which changes will increase the equilbrium concentration of HgI₄²¯?

I Increasing the mass of HgO(s) present
II Increasing [I¯]
III Adding 1 M HCI

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

32. The equilibrium constant for this reaction is approximately 10¯³.

HPO₄²¯(aq) + HCO₃¯(aq) <===> H₂PO₄¯(aq) + CO₃²¯(aq)

Which is the strongest conjugate base in this reaction?

(A) HPO₄²¯(aq)
(B) HCO₃¯(aq)
(C) H₂PO₄¯(aq)
(D) CO₃²¯(aq)

33. A weak acid, HX, has K_a = 9.0 x 10¯⁶. Within which range does the percent dissociation for a 0.01 M HX solution lie?

(A) between 1.0% and 10%
(B) between 0.10% and 1.0%
(C) between 0.01% and 0.10%
(D) less than 0.01%

34. Which salt dissolves in water to produce a solution with a pH < 7?

(A) NaC₂H₃O₂
(B) Ba(NO₃)₂
(C) AlCl₃
(D) KClO₄

35. Which mixture forms a buffer when dissolved in 1.0 L of water?

(A) 0.2 mol NaOH + 0.2 mol HBr
(B) 0.2 mol NaCl + 0.3 mol HCl
(C) 0.4 mol HNO₂ + 0.2 mol NaOH
(D) 0.5 mol NH₃ + 0.5 mol HCl

36. The equilibrium constant for this reaction is 3.6 x 10¯⁷

OCl¯(aq) + H₂O(l) <===> HOCl(aq) + OH¯(aq)

What is K_a for HOCl?

(A) 2.8 x 10¯⁸
(B) 3.6 x 10¯⁷
(C) 6.0 x 10¯⁴
(D) 2.8 x 10⁶

37. For which titration would the use of phenophthalein introduce a significant error?

K_indicator for phenolphthalein = 1 x 10¯⁹

38. When solid lead iodide is added to water, the equilibrium concentration of I¯ becomes 2.6 x 10¯³ M. What is the K_sp for PbI₂?

(A) 2.2 x 10¯⁹
(B) 8.8 x10¯⁹
(C) 1.8 x 10¯⁸
(D) 3.5 x 10¯⁸

39. When these standard half reactions are combined to give a spontaneous reaction in a voltaic cell, what is the cell voltage?

(A) 1.74 V
(B) 1.46 V
(C) 0.94 V
(D) 0.66 V

40. How many moles of elections are transferred when this equation is balanced with the smallest whole number coefficients?

_IO₃¯ + _H₂O₂ + _H⁺ ---> _I₂ + _O₂ + _H₂O

(A) 2
(B) 4
(C) 5
(D) 10

41. If the E°_cell for a given reaction has a negative value, which gives the correct relationships for the values of DG° and K_eq?

A. DG° > 0; K_eq < 1
B. DG° > 0; K_eq > 1
C. DG° < 0; K_eq > 1
D. DG° < 0; K_eq < 1

42. When anions undergo oxidation, they move toward the

A. anode of a voltaic cell and the cathode of an electrolytic cell
B. cathode of a voltaic cell and the anode of an electrolytic cell
C. anodes of both types of cells
D. cathodes of both types of cells

43. How much time is required to produce 0.10 mol of chlorine gas during the electrolysis of molten sodium chloride using a current of 3.0 amps?

A. 55 min
B. 110 min
C. 220 min
D. 330 min

44. The visible spectrum of the hydrogen atom consists of a series of lines that

A. converge at low energies.
B. converge at high energies.
C. are separated by equal energies.
D. are randomly spaced.

45. When the elements C, N and Si are arranged in order of increasing first ionization energy, which is the correct order?

A. C < N < Si
B. N < C < Si
C. Si < C < N
D. Si < N < C

46. Which is a possible set of quantum numbers for a valence electron in ground state atom of phosphorus?

	n	l	ml	ms
A.	2	1	0	+1/2
B.	3	0	0	0
C.	3	1	-1	+1/2
D.	3	2	1	-1/2

47. These are the first eight ionization energies for a particular neutral atom. All values are expressed in MJ mol¯¹. How many valence electrons does this atom possess?

A. 1
B. 2
C. 4
D. 6

48. Which gaseous ion in its ground state has the greatest number of unpaired electrons?

A. Mn³⁺
B. Fe³⁺
C. Co³⁺
D. Ni³⁺

49. Which of these characteristics describe the PCl₃ molecule?

I trigonal planar shape
II sp³ hybridized phosphorus atom
III polar bonds
IV nonpolar molecule

A. I and II only
B. II and III only
C. I and IV only
D. I, III, and IV only

50. According to the Lewis structure for the HNNH, how many sigma bonds, pi bonds, and lone pairs of electrons are present?

	sigma bonds	pi bonds	lone pairs of electrons
A.	2	2	2
B.	3	2	0
C.	3	1	2
D.	3	0	4

51. In the Lewis structure for the BrF₄¯ ion, how many lone pairs of electrons are placed around the central atom?

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

52. What term is used for the measure of an atom's attraction for the electrons that constitute a covalent bond?

(A) ionization energy
(B) bond dissociation energy
(C) electron affinity
(D) electronegativity

53. One way of writing the Lewis structure of the cyanate ion, OCN¯, places one double bond between the carbon atom and the oxygen atom and another double bond between the carbon atom and the nitrogen atom. What are the formal charges on the oxygen, carbon, and nitrogen atoms, respectively for this structure?

(A) 0, 0, -1
(B) -1, 0, 0
(C) -1, +1, -1
(C) -2, 1, 0

54. Lithium crystallizes in a body-centered cubic unit cell. What is the mass of one unit cell? Report your answer in grams.

(A) 1.15 x 10¯²³ g
(B) 2.30 x 10¯²³ g
(C) 5.75 x 10¯²³ g
(D) 1.04 x 10¯²² g

55. Which compound is an isomer of 2-methylbutane?

(A) 2,2-dimethylpropane
(B) 2,2-dimethylbutane
(C) 2-methylpentane
(D) 2-metylbutene

56. How many different isomers have the molecular formula C₃H₆Cl₂?

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

57. Which type of compound is most likely to be colored?

(A) acid
(B) chlorofluorocarbon
(C) ketone
(D) polyene

58. A certain four-carbon alcohol reacts with acidified potassium dichromate to form a ketone. Which structure is most likely for that four-carbon alcohol?

(A) H₃CCH₂CH₂CH₂OH
(B) H₃CCH₂CH(OH)CH₃
(C) H₃CC(OH)(CH₃)₂
(D) H₂C=CHCH₂CH₂OH

59. What type of compound is CH₃CH₂OCH₃?

(A) alcohol
(B) aldehyde
(C) ester
(D) ether

60. Which pair are two different names for the same compound?

(A) acetic acid and ethanoic acid
(B) formaldehyde and methanol
(C) isopropyl alcohol and 1-propanol
(D) ethene and ethyne