U.S. National Chemistry Olympiad

1995 National Test

Notes


1. Which 0.1 M solution will form a precipitate with excess 0.1 M KOH but not with excess 0.1 M NH3?

(A) AgNO3
(B) AlCl3
(C) BaCl2
(D) MgCl2

2. The members of which pair are most similar in color?

(A) chlorine, Cl2(g), and bromine, Br2(l)
(B) silver chloride, AgCl(s), and silver sulfide, Ag2S(s)
(C) copper(II) chloride, CuCl2(s), and calcium chloride, CaCl2(s)
(D) potassium permanganate, KMnO4(aq), and iodine, I2(g)

3. Which compound is expected to be the least soluble in water?

(A) CH3COOH
(B) CH3COONa
(C) CH3OCH3
(D) CH3CH2OH

4. When a small amount of solid calcium phosphide, Ca3P2, is added to water, what are the most likely products?

(A) aqueous Ca2+ and OH¯ ions and gaseous PH3
(B) aqueous Ca2+ and OH¯ ions and aqueous H3PO3
(C) solid CaH2 and aqueous H3PO3
(D) solid CaO and gaseous PH3

5. A student plans to calculate the molar mass of an unknown gas from the mass of a measured volume of gas at a known temperature and pressure. To how many significant figures should the student report the molar mass if 175.0 mL of the gas weighs 1.0315 g at a temperature of 22 °C and pressure of 742 mmHg?

(A) two
(B) three
(C) four
(D) five

6. The concentration of a colored substance is to be determined by measuring the absorbance of an aqueous solution of it and interpolating from a graph of absorbance versus concentration. Which procedural error will result in a concentration that is too high?

Procedural Mistakes

I. rinsing the cuvette with water just before filling it with the unknown solution
II. measuring the absorbance of the unknown solution at a wavelength other than its maximum
III. using a cuvette for the unknown solution that has a slightly longer path length than you thought it had

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

7. What is the density (in g mL¯1) of a 3.60 M aqueous sulfuric acid solution that is 29.0% H2SO4 by mass?

(A) 1.22
(B) 1.45
(C) 1.64
(D) 1.88

8. Calculate the volume of 0.110 M copper(II) nitrate solution that can be prepared by dissolving 33.29 g of copper(II) nitrate trihydrate in sufficient distilled water.

The molar mass of Cu(NO3)2 . 3 H2O equals 241.6 g mol¯1

(A) 0.50 L
(B) 1.25 L
(C) 2.50 L
(D) 3.75 L

9. In alkaline solution, chlorine disproportionates to form chloride, Cl¯, and chlorate, ClO3¯, ions. What is the ratio of chloride ions to chlorate ions formed?

(A) one to one
(B) three to one
(C) five to one
(D) seven to one

10. What is the formula of a substance with mass percentages of 35.97% for S, 62.92% for O and 1.13% for H?

(A) H2SO3
(B) H2SO4
(C) H2S2O7
(D) H2S2O8

11. When 1.20 g of sulfur is melted with 15.00 g of naphthalene, the solution freezes at 77.2 C. What is the molar mass of this form of sulfur.
Data for Naphthalene
Melting point, m.p. 80.00 °C
Freezing point depression
constant, kf
-6.80 °C m¯1

(A) 180 g mol¯1
(B) 190 g mol¯1
(C) 260 g mol¯1
(D) 450 g mol¯1

12. Antimony reacts with chlorine according to this equation:

2 Sb + 3 Cl2 <===> 2 SbCl3

How many grams of SbCl3can be prepared if 0.0120 mol of antimony are reacted with 0.0200 mol of chlorine?

The molar mass of SbCl3 equals 228.2 g mol¯1

(A) 1.52 g
(B) 1.83 g
(C) 2.74 g
(D) 4.56 g

13. The pressure on a sample of water at its triple point is reduced while the temperature is held constant. Which phase changes are favored?

I. fusion
II. sublimation
III. vaporization

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

14. A face-centered cubic unit cell of aluminum contains the equivalent of how many atoms?

(A) one
(B) two
(C) three
(D) four

15. Which statement about the composition of the vapor over an ideal 1:1 molar mixture of benzene and toluene is correct? Assume the temperature is constant at 25 °C.
Vapor Pressure Data, 25 °C
benzene 75 mmHg
toluene 22 mmHg

(A) The vapor will contain a higher percentage of benzene
(B) The vapor will contain a higher percentage of toluene.
(C) The vapor will contain equal amounts of benzene and toluene.
(D) Not enough information is given to make a prediction.

16. An open-ended mercury manometer is used to measure the pressure exerted by a trapped gas as shown in the figure. Atmospheric pressure is 749 mmHg. What is the pressure of the trapped gas?

(A) 292 mmHg
(B) 457 mmHg
(C) 749 mmHg
(D) 1041 mmHg

17. Which gas will have the greatest density at 25 °C and 1 atm pressure?

(A) O3
(B) SiH4
(C) PH3
(D) C3H8

18. By what ratio will the average velocity of the molecules in a gas change when the temperature is raised from 50 °C to 200 °C?

(A) 1.21 / 1
(B) 1.46 / 1
(C) 2 / 1
(D) 4 / 1

19. When a liquid evaporates, which is true about the signs of the enthalpy and entropy changes?

  Δ H Δ S
(A) + +
(B) + -
(C) - +
(D) - -

20. Which substance would be expected to have the highest absolute entropy at 25 °C?

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

21. Calculate the enthalpy of formation, ΔHf, for C2H5OH from the tabulated data and its heat of combusion as represented by this equation.

Enthalpy of Formation Data
H2O(g) - 241.8 kJ-mol¯1
CO2(g) - 393.5 kJ-mol¯1

C2H5OH (l) + 3 O2 (g) ---> 3 H2O (g) + 2 CO2 (g)

The ΔH for this reaction equals - 1234.7 kJ

(A) -2747.1 kJ-mol¯1
(B) -277.7 kJ-mol¯1
(C) 277.7 kJ-mol¯1
(D) 2747.1 kJ-mol¯1

22. Which of these species has a standard enthalpy of formation equal to zero?

(A) F2(g)
(B) F(g)
(C) HF(aq)
(D) F¯(aq)

23. Under which circumstance would the free energy change for a reaction be relatively temperature independent?

(A) ΔH° is negative.
(B) ΔH° is positive.
(C) ΔS° has a large positive value.
(D) ΔS° has a small value.

24. Use the free energy change, ΔG° = + 63.3 kJ, for this reaction to calculate the Ksp of Ag2CO3(s) in water at 25 °C.

Ag2CO3 (s) <===> Ag+ (aq) + CO32¯ (aq)

(A) 3.2 x 10¯26
(B) 8.0 x 10¯12
(C) 2.9 x 10¯3
(D) 7.9 x 10¯2

25. CH3Cl(g) + H2O (g) ---> CH3OH (g) + HCl (g)

When this reaction was studied, these data were obtained.

  Initial Concentration, M Initial Rate, M s¯1
  CH3Cl H2O
Exp. 1 0.100 0.100 0.182
Exp.2 0.200 0.200 1.45
Exp.3 0.200 0.400 5.81

Based on these data, what are the orders of the two reactants in this reaction?

  CH3Cl H2O
(A) 1 1
(B) 2 1
(C) 1 2
(D) 2 2

26. The activity of a radioactive isotope is 3000 counts per minute at a certain time and 2736 counts per minute 48 hours later. What is its half-life?

(A) 831 h
(B) 521 h
(C) 361 h
(D) 1.44 h

27. Which function of [X], plotted against time, will give a straight line for a second order reaction?

(A) [X]
(B) [X]2
(C) ln[X]
(D) 1/[X]

28. Two reactions with different activation energies have the same rate at room temperature. Which statement correctly describes the rates of these two reactions at the same higher temperature?

(A) The reaction with the greater activation energy will be faster.
(B) The reaction with the smaller activaiton energy will be faster.
(C) The two reactions will have the same rates.
(D) A prediction cannot be made without further information.

29. For an exothermic reaction in which 2 mol of gaseous reactants are converted to 3 mol of gaseous products, which will lower the reaction rate?

(A) decreasing the volume
(B) decreasing the temperature
(C) removing some of the products
(D) adding a catalyst

30. For the hypothetical reaction 2 X + 2 G ---> Q + 2 M, the rate expression is d[Q] / dt = k[X]2[G]. Which mechanism is most likely?

(A) 2 X + GT <===> 2Q + R (fast equilibrium)
  Q + R + G ---> 2 M (slow, rate determining)
(B) X + G <===> Q + R (fast equilibrium)
  R + X ---> 2 M (slow, rate determining)
(C) X + X <===> X2 (fast equilibrium)
  X2 + G ---> Q + T (slow, rate determining)
  T + G ---> 2M (fast)
(D) G + G <===> G2 (fast equilibrium)
  G2 + X ---> Q + T (slow, rate determining)
  T + X ---> 2M (fast)

31) A saturated solution of MgF2 contains 1.16 x 10¯3 mol of MgF2 per liter. What is the Ksp of MgF2?

(A) 2.7 x 10¯6
(B) 1.6 x 10¯9
(C) 3.1 x 10¯9
(D) 6.2 x 10¯9

32) In which reaction will an increase in the volume of the container favor the formation of products?

(A) C(s) + H2O(g) <===> CO(g) + H2(g)
(B) H2(g) + I2(g) <===> 2 HI(g)
(C) 4 NH3(g) + 5 O2(g) <===> 4 NO(g) + 6 H2O(l)
(D) 3 O2(g) <===> 2 O3(g)

33)
Reaction   K  
1/2 N2(g) + O2(g) <===> NO2(g) K1
2 NO2(g) <===> N2O4(g) K2

Given these reactions and their equilibrium constants, write an expression for K for this reaction.

N2O4(g) <===> N2(g) + 2O2(g)

(A) K1K2
(B) (K2)2K2
(C) 1/ [K2(K2)2]
(D) 1/ [(K1)2K2]

34) Consider this reaction, whose Kc = 33.

SO2(g) + NO2(g) <===> SO3(g) + NO(g)

If 0.10 mol each of SO2 and NO2are placed in a 1.0 L container, what is the concentration of SO2 at equilibrium?

(A) 0.0030 M
(B) 0.015 M
(C) 0.085 M
(D) 0.097 M

35) Consider this reaction.

2 SO2(g) + O2(g) <===> 2 SO3(g)

The equilibrium constant may be stated in terms of Kc or Kp. How are they related for this reaction?

(A) Kp = Kc
(B) Kp = Kc(RT)
(C) Kp = Kc(RT)2
(D) Kp = Kc(RT)-1

36) What will be the result if 100 mL of 0.06 M Mg(NO3)2 is added to 50 mL of 0.06 M Na2C2O4?

The Ksp of MgC2O4(s) equals 8.6 x 10¯5

(A) No precipitate will form.
(B) A precipitate will form and an excess of Mg2+ ions will remain in the solution
(C) A precipitate will form and an excess of C2O42¯ ions will remain in the solution.
(D) A precipitate will form and but neither ion is present in excess.

37) Calculate the percentage of pyridine, C5H5N, that forms pyridinium ion, C5H5NH+, in a 0.10 M aqueous pyridine solution (Kb for C5H5N = 1.7 x 10¯9)

(A) 0.0060%
(B) 0.013%
(C) 0.77%
(D) 1.6%

38) The conjugate acid and conjugate base of HSO4¯ are, respectively,

(A) H3O+ and OH¯
(B) H3O+ and SO42¯
(C) H2SO4 and OH¯
(D) H2SO4 and SO42¯

39) When the acids: HClO3, H2SO3, H3BO3 are arranged in order of increasing stength (weakest acid first), the correct order is

(A) HClO3 < H2SO3 < H3BO3
(B) H2SO3 < H3BO3 < HClO3
(C) HClO3 < H3BO3 < H2SO3
(D) H3BO3 < H2SO3 < HClO3

40) Which combinations of equal volumes of solution will result in buffer solutions?

I. 0.1 M HCl and 0.1 M NH3
II. 0.1 M HNO2 and 0.05 M NaOH
III. 0.05 M HNO2 and 0.05 NH3

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

41) A solution contains 20 mmol H3PO4 and 10 mmol NaH2PO4. What many mmol of NaOH must be added to convert all the phosphate to HPO4¯?

(A) 30 mmol
(B) 50 mmol
(C) 60 mmol
(D) 80 mmol

42) Which represents an acid-base reaction according to the Lewis definition but not according to the Brønsted-Lowry definition?

(A) MgCl2(s) <===> Mg2+(aq) + 2 Cl¯(aq)
(B) CN¯(aq) + H2O(l) <===> HCN(aq) + OH¯(aq)
(C) NH3(g) + HBr(g) <===> NH4Br(s)
(D) Fe(H2O)63+(aq) + H2O(l) <===> [Fe(H2O)5OH]2+(aq) + H3O+(aq)

43) If an aqueous sodium chloride solution is electrolyzed, which phenomenon is not observed?

(A) The concentration of [Na+] decreases.
(B) Cl2(g) is produced atthe anode.
(C) the solution becomes more basic.
(D) H2(g) is produced at the cathode.

44)
Standard Reduction Potentials
Pb2+(aq) + 2 e¯ ---> Pb(s) - 0.127 V
Cu+(aq) + e¯ ---> Cu(s) +0.518 V

Which statement is true about a galvanic cell employing Pb, Cu, Pb2+, and Cu+?

(A) The cell potential increases when the Cu+ solution is diluted.
(B) Anions flow from the lead half-cell to the copper half-cell through the salt bridge.
(C) Twice as many electrons pass through the lead electrode as through the copper.
(D) The concentration of the cation in the cathode compartment changes faster than the cation concentration in the anode compartment.

45) Consider this reaction, for which E° = 0.617 V.

Sn2+ + 2 Fe3+ ---> Sn4+ + 2 Fe2+

What is the value of E when [Sn2+] and [Fe3+] = 0.50 M and [Sn4+] and [Fe2+] = 0.10 M

(A) 0.699 V
(B) 0.679 V
(C) 0.658 V
(D) 0.576 V

46) What is the value of the cell potential, E°, for this reaction?

Hg(l) + 2 Co3+(aq) ---> 2 Co2+(aq) + Hg<2+(aq)
Reaction
Hg2+(aq) + 2e- ---> Hg(l) +0.86 V
Co3+(aq) + e- ---> Co2+(aq) +1.82 V

(A) 0.96 V
(B) 2.68 V
(C) 2.78 V
(D) 5.50 V

47) An oxidation-reduction reaction in which 3 electrons are transferred has a ΔG° of +18.55 kJ at 25 °C. What is value of E°?

(A) -0.064 V
(B) +0.192 V
(C) -0.028 V
(D) -0.192 V

48) A vanadium electrode is oxidized electrically. If the mass of the electrode decreases by 114 mg during the passage of 650. coulombs, what is the oxidation state of the vanadium product?

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

49) Calculate the wavelength of light required to break the bond between two chlorine atoms in a chlorine molecule. The Cl-Cl bond energy is 243 kJ mol¯1

(A) 8.18 x 10¯31 m
(B) 6.26 x 10¯21 m
(C) 4.93 x 10¯7 m
(D) 4.11 x 10¯6 m

50) 90-Th-232 is coverted to 82-Pb-208 by the emmision of a series of alpha and beta particles. How many alpha and beta particles are emitted in this process?

  alpha beta
(A) 3 2
(B) 4 8
(C) 5 2
(D) 6 4

Use these ionization energies for questions 51 and 52.
Ionization Energies, kJ mol¯1
Element 1st 2nd 3rd 4th 5th 6th 7th 8th
A 1.7 3.4 6.0 8.4 11.0 15.0 17.8 92.0
B 0.50 4.6 6.9 9.5 13.3 16.6 20.0 25.5
C 0.56 1.8 2.7 11.6 15.0 18.3 23.0 27.0
D 0.74 1.5 7.7 10.5 13.6 18.0 22.0 26.0

51) Which element is most likely to form an ion with a +3 charge?

(A) A
(B) B
(C) C
(D) D

52) Which element is most likely to form a stable anion?

(A) A
(B) B
(C) C
(D) D

53) The species F¯, Ne and Na+ all have the same number of electrons. Which is the correct order when they are arranged in order of decreasing size (largest first)?

(A) F¯ > Ne > Na+
(B) Ne > Na+ > F¯
(C) Na+ > F¯ > Ne
(D) F¯ > Na+ > Ne

54) Which electron configuration is not allowed for either a neutral atom or an ion in its ground state?

(A) 1s2 2s22p3
(B) 1s2 2s12p6
(C) 1s2 2s22p6 3s23p63d3
(D) 1s2 2s22p6 3s23p63d10 4s1

55) Which combination will give the strongest ionic bond?

(A) K+ and Cl¯
(B) K+ and O2¯
(C) Ca2+ and Cl¯
(D) Ca2+ and O2¯

56) Which plot best represents the potential energy of two hydrogen atoms as they approach one another to form a hydrogen molecule, H2?

57) Which of these molecules have nonbonding electron pairs on the central atom?

I. SF4
II. ICl3
III. SO2

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

58) According to the Valence-Bond Theory, what are the states of hybridization of the carbon atoms (reading from left to right) in the compound CH2CHCO2H?

(A) sp2, sp, sp3
(B) sp2, sp2, sp2
(C) sp3, sp2, sp3
(D) sp3, sp3, sp3

59) The O-Si-O bond angles in SiO2 (quartz) are closest to

(A) 180
(B) 120
(C) 110
(D) 100

60) Which formula(s) could represent more than one compound?

I. CH2Cl2
II. Pt(NH3)2Cl2

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

Free Response

1. (12%) The operation of automobile airbags depends on the rapid decomposition of sodium azide, NaN3, to its elements.

a) Write a balanced equation for this decomposition.
b) If an airbag has a volume of 5.2 x 104 cm3, how many moles of nitrogen gas would be required to fill it at 25 °C and 1.5 atm pressure?
c) How many grams of sodium azide would be needed to provide this volume of nitrogen.
d) If this decomposition is complete in 40 milliseconds, calculate its average rate in liters of nitrogen per second.

2. (14%) A 50.00 mL sample of a 0.120 M solution of HOBr is titrated with 0.100 M solution of KOH. After the addition of 25.00 mL of KOH, the pH measured is 8.45.

a) Calculate Ka for HOBr.
b) Calculate the pH of the solution initially (i.e. before any base has been added)
c) Calculate the [OH¯] at the equivalence point of the titration.
d) If an additional 0.50 g of NaOBr were added to the neutralized solution, describe qualitatively (and explain your answer) what would happen to the numerical value of
i. pH
ii. Ka

3. (15%) Solid tin dioxide, SnO2, can be reacted with either hydrogen gas or solid carbon to form metallic tin and water vapor or carbon dioxide gas, respectively, as by-products. Consider these data for thermodynamic values at 25 °C.

Substance ΔHf (kJ mol¯1) S° (kJ mol¯11)
C(s) -- 0.0057
CO2(g) - 393.5 0.2136
H2(g) -- 0.1310
H2O(g) - 241.8 0.1887
Sn(s) -- 0.0516
SnO2(s) - 580.7 0.0523
a) Calculate the values of ΔH° and ΔS° at 25 °C for each of these reactions.
i. SnO2(s) with H2(g)
ii. SnO2(s) with C(s)
b) Calculate the temperature at which each of these processes becomes spontaneous.
c) Based on economic principles, which process is preferred? Explain.

4. (8%) The reaction between bromate ions and bromide ions in acid solution, represented by this equation.

BrO3¯(aq) + 5 Br¯(aq) + 6 H+(aq) ---> 3 Br2(aq) + 3 H2O(l)

Consider these experimental data.

[BrO3¯], mol L¯1 [Br¯], mol L¯1 [H+], mol L¯1 Δ[Br ¯] / Δt),
mol L¯11
0.200 0.200 0.200 1.04 x 10¯2
0.400 0.200 0.200 2.08 x 10¯2
0.400 0.400 0.200 4.16 x 10¯2
0.200 0.200 0.400 4.16 x 10¯2
a) Determine the order of this reaction with regard to
i. BrO3¯
ii. Br¯
iii. H+
b) Determine the value of the specific rate constant and give its units.
c) Account for the fact that the orders of the reactants are not the same as the coefficients in the balanced equation.

5. (14%) Write net equations for the following reactions. Use appropriate ionic and molecular formulas for the reactants and products and omit formulas for all ions or molecules that do not take part in a reaction. Write structural formulas for all organic substances. You need not balance the equations. All reactions occur in aqueous solution unless otherwise indicated.

a) Concentrated phosphoric acid is added to solid sodium bromide.
b) Solutions of barium hydroxide and copper(II) sulfate are mixed.
c) Boron trichloride is bubbled through excess water.
d) Lead sulfide is burned in air.
e) A solution of potassium dichromate is added to an acidic solution of tin(II) chloride.
f) Excess aqueous ammonia is added to a solution of cobalt(II) chloride.
g) Solid aluminum nitrate is added to water.

6. (11%) Carbon dioxide, CO2, dissolves in water to give a mixture of CO32¯ and HCO3¯.

a) Write Lewis electron-dot structures for CO232¯, and for HCO3¯. Show all valence electrons.
b) Arrange the eight carbon-to-oxygen bonds in CO2, CO32¯, and HCO3¯ in order of increasing length. Specify which bonds are equal in length. Give reasons for your ordered arrangement.
c) Consider CO2, CO32¯, and HCO3¯. For each species, give the hybridization of the carbon atom and the oxygen-to-carbon-to-oxygen bond angle.

7. (14%) The melting and boiling points of several nitrogen oxides are given at standard pressure of 1 atm.

Substance Melting Point, °C Boiling Point, °C
NO -163.6 -151.8
N2O -90.8 -88.5
NO2 -11.2 21.2
N2O3 -102 3.5 (decomposes)
N2O5 -30. 47 (decomposes)
a) Provide a molecular level explanation for the general trends in these melting and boiling points.
b) Identify the compound that does not follow this general trend and outline your reasoning.
c) Define what is meant by ideal behavior for a gas and identify two factors that cause gases to deviate from such behavior.
d) Identify the nitrogen oxides that are gases at 0 °C, and select the one that is least likely to behave ideally. Explain your choice in terms of the factors in (c)

8. (12%) Provide atomic or molecular-level explanations to account for these experimental observations.

a) Calcium forms a solid dichloride which is colorless but chromium forms both a solid dichloride and a solid trichloride, both of which are colored.
b) Oxygen forms a dichloride but sulfur forms both a dichloride and a tetrachloride.
c) The melting points of the sodium and magnesium halides decrease with increasing atomic mass of the halide, but the melting points of the halides of silicon, phosphorus, and sulfur increase with increasing atomic mass of the halide.