U.S. National Chemistry Olympiad 1995 National Test

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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?

(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.

(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.

(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?

	[delta] H	[delta] 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, [delta]H_f, for C₂H₅OH from the tabulated data and its heat of combusion as represented by this equation.

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?

(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) [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 K_sp 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.

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 K_sp 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)

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 K_c = 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 K_c or K_p. How are they related for this reaction?

(A) K_p = K_c
(B) K_p = K_c(RT)
(C) K_p = K_c(RT)²
(D) K_p = K_c(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 K_sp 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 (K_b 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)

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)

(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?

(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.

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