Chemistry 1311
Exam 4, Fall, 2002

I. Multiple Choice (105 points) Select the best response for each of the following and record on the scantron. Do not erase; erasures may result in a misgrading of your response.

1. The critical temperature and pressure of ammonia are 132C and 111 atm respectively. Under which of the following conditions will ammonia exist as a liquid?
a. 133C and 111 atm
b. 132C and 100 atm
c. 131C and 1 atm
d. 131C and 111 atm
e. none of the above, ammonia cannot exist as a liquid

2. Which of the following statements correctly correlates intermolecular forces with the observed property of a liquid? Molecular systems with
a. strong intermolecular forces have low critical temperatures.
b. weak intermolecular forces have low vapor pressures.
c. strong intermolecular forces have low enthaplies of vaporization.
d. weak intermolecular forces have low viscosities.
e. strong intermolecular forces have low melting points.

3. Real gases have nearly ideal gas behavior at
a. high temperatures and low pressures.
b. high temperatures and high pressures.
c. low temperatures and low pressures.
d. low temperatures and high pressures.
e. large numbers of moles and small volumes.

4. Hydrogen bonding occurs
a. between all molecules containing a hydrogen atom.
b. between a hydrogen atom (covalently bonded to a N, O, or F) on one molecule and a N, O, or F of another molecule.
c. between a N, O, or F on one molecule and any or all hydrogens on another molecule.
d. between H2O and CH4 molecules.
e. between all molecules having a permanent dipole.

5. CH3OH (skeletal structure is HCOH) has a higher boiling point than CH3OCH3 (skeletal structure HCOCH) because
a. CH3OH has dipole-dipole forces and CH3OCH3 does not.
b. CH3OH has stronger London forces than CH3OCH3.
c. CH3OH has lower molar mass than does CH3OCH3.
d. CH3OH has hydrogen bonding intermolecular forces and CH3OCH3 does not.
e. CH3OCH3 has London intermolecular forces and CH3OH does not.

6. Equal, but separate, volumes of Ar, CO2, and N2 exist at the same temperature and pressure. What is true about the relationships between the gases?
a. The masses of the three gases are equal.
b. The number of molecules of the three gases are equal.
c. The number of moles of gas increase from N2 to Ar to CO2.
d. The number of moles of gas increase from CO2 to Ar to N2.
e. The masses of gas increase from CO2 to Ar to N2.

7. The boiling point of several substances are listed. Which of the substances has the highest vapor pressure at a given laboratory temperature?
a. I2: b.p.= 184C
b. HF: b.p. = 20C
c. NH3; b.p. = 33C
d. CCl4; b.p. = 77C
e. C6H6: b.p.= 80C

8. Following are four properties of gases:
i. gases diffuse
ii. gas molecules exert pressure
iii. gas molecules are in rapid, random, straight‑line motion
iv. the average kinetic energy of gas molecules is constant at a given temperature
Which of these four statements are also true of solids?
a. ii and iv only
b. iii and iv only
c. ii, iii, and iv only
d. i and iii only
e. i, ii, iii, and iv

9. The phase diagram (P vs. T) at right is for a pure substance. Identify the point at which equilibrium exists between the liquid and vapor phases of the substance.
a. A
b. B
c. C
d. D
e. E

10. A gas is confined to a vessel at a given pressure and temperature. The gas is then transferred to a vessel where the volume of the gas is tripled (in milliliters) and the temperature of the gas is increased by a factor of 4 (in kelvins). By what factor has the pressure changed for the gas?
a. no change
b. decreased by a factor of 12
c. decreased by a factor of
d. increased by a factor of 12
e. increased by a factor of 4/3

11. Identify the molecule having the greatest velocity at a given temperature.
a. CO2
b. N2
c. H2O
d. CO
e. all have the same velocity

12. Two flasks of volumes V1 and V2 (V2>V1) contain the same number of helium atoms at the same temperature. Which of the following statement(s) is true?
a. The average velocity of the helium atoms in flask 1 is greater than those in flask 2.
b. The pressure exerted by the helium gas in flask 1 is greater than that in flask 2.
c. The pressure exerted by the helium gas in flask 1 equals that of flask 2.
d. The average kinetic energy of the helium atoms in flask 1 is lower than those of flask 2.
e. The intermolecular forces for the helium atoms in flask 2 is greater than those of flask 1.

13. Which of the following molecules has the predicted highest boiling point?
a. F2
b. Cl2
c. ClF
d. BrF
e. BrCl

14. Water beads on a wax surface. The best explanation for this observation is that
a. the cohesive forces between water molecules are greater than the adhesive forces between water and wax.
b. the cohesive forces between water molecules are less than the adhesive forces between water and wax.
c. the wax molecules have a greater viscosity that water molecules.
d. the surface tension of the wax molecules is greater than that of the water molecules.
e. avocados are green and (most) apples are red.

15. Which statement regarding the effect of temperature on properties of liquids is incorrect? Increasing the temperature of a liquid
a. increases its vapor pressure.
b. decreases its surface tension.
c. increases its evaporation rate.
d. increases its standard enthalpy of vaporization.
e. decreases its viscosity.

16. Which of the following is not a match of the type of the solid for the corresponding substance?
a. covalent or network solid, C(graphite)
b. ionic solid, CuCl2
c. ionic solid, copper
d. molecular solid, CO2
e. metallic solid, gold

17. Tomorrow the Javelina football team participates in the 1st round of the NCAA-II playoffs. Game time is at noon and their opponents are from
a. the University of Texas
b. Bishop High School
c. the Texas Academy of Chemists
d. Texas A&M University at College Station
e. the University of Nebraska at Kearney

II. Exercises (60 points). The organization of all exercises must be shown clearly and legibly in order to receive any partial credit. Correct answers without a logical and systematic analysis and problem setup will receive only 10% credit.

1. (25 points) A 700.0 mL volume of SO2(g) at 1.40 atm and 27.0C is mixed with a 500.0 mL volume of O2(g) at 0.800 atm also at 27.0C. The following reaction occurs:
2 SO2(g) + O2(g) 2 SO3(g)



a. (15 pts) What is the partial pressure of SO3(g) when the reaction is complete? Assume a theoretical yield.

O2 is the limiting reactant; pso3 = 0.667 atm

b. (10 pts) Identify the excess reactant and determine its partial pressure.

SO2 is the excess reactant (by 7.30 x 10-3 mol); pso2 = 0.150 atm

2. (35 points) A 0.589 g sample of an unknown gas is confined to a 300 mL reaction vessel at a pressure of 500 torr and a temperature of 44C.
a. (10 pts) What pressure will the gas exert if the gas is transferred to a 1500 mL vessel at 22C?

93.1 torr or 0.123 atm

b. (10 pts) What is the molar mass of the gas?

77.6 g/mol

c. (5 pts) What is the density of the gas at STP?

3.47 g/L at STP

d. (10 pts) Will the diffusion rate of the gas be greater than or less than the rate of diffusion of xenon (molar mass = 131 g/mol)? By what factor? Your response must be substantiated!!

The diffusion rate of the gas will be greater by a factor of 1.30

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