U.S. National Chemistry Olympiad 1995 National
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1. Which 0.1 M solution will form a precipitate with excess 0.1 M KOH but not with
excess 0.1 M NH3?
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?
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?
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
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
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?
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
(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?
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.
Freezing point depression
-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?
(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
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
(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?
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
20. Which substance would be expected to have the highest absolute entropy at 25 °C?
21. Calculate the enthalpy of formation, [delta]Hf, for C2H5OH
from the tabulated data and its heat of combusion as represented by this equation.
|Enthalpy of Formation Data
- 241.8 kJ-mol¯1
- 393.5 kJ-mol¯1
C2H5OH (l) + 3 O2 (g)
---> 3 H2O (g) + 2 CO2 (g)
The [delta]H 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?
23. Under which circumstance would the free energy change for a reaction be relatively
(A) [delta]H° is negative.
(B) [delta]H° is positive.
(C) [delta]S° has a large positive value.
(D) [delta]S° has a small value.
24. Use the free energy change, [delta]G° = + 63.3 kJ, for this reaction to calculate
the Ksp of Ag2CO3(s) in water at 25 °C.
Ag2CO3 (s) <===> Ag+ (aq) + CO32¯
(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
Based on these data, what are the orders of the two reactants in this
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
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?
||2 X + GT <===> 2Q + R (fast equilibrium)
||Q + R + G ---> 2 M (slow, rate determining)
||X + G <===> Q + R (fast equilibrium)
||R + X ---> 2 M (slow, rate determining)
X + X <===> X2 (fast equilibrium)
X2 + G ---> Q + T (slow, rate determining)
||T + G ---> 2M (fast)
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)
1/2 N2(g) + O2(g) <===> NO2(g)
2 NO2(g) <===> N2O4(g)
Given these reactions and their equilibrium constants, write an expression for K for
N2O4(g) <===> N2(g) + 2O2(g)
(C) 1/ [K2(K2)2]
(D) 1/ [(K1)2K2]
34) Consider this reaction, whose Kc = 33.
SO2(g) + NO2(g) <===> SO3(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
(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)
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)
43) If an aqueous sodium chloride solution is electrolyzed, which phenomenon is not
(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.
|Standard Reduction Potentials
Pb2+(aq) + 2 e¯ ---> Pb(s)
||- 0.127 V
Cu+(aq) + e¯ ---> Cu(s)
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)
Hg2+(aq) + 2e- ---> Hg(l)
Co3+(aq) + e- ---> Co2+(aq)
(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
[delta]G° 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?
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?
Use these ionization energies for questions 51 and 52.
Ionization Energies, kJ mol¯1
51) Which element is most likely to form an ion with a +3 charge?
52) Which element is most likely to form a stable anion?
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
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?
(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
60) Which formula(s) could represent more than one compound?
(A) I only
(B) II only
(C) both I and II
(D) neither I nor II