Chem 128, Exam I
February 21, 2003
Name
I. (41 points)
A. (12 points) Write your answers on the blanks provided.
1. Which of the following solutes would be more soluble in water?
a. CH3 OH or C17 H35 OH
b. C2 H5 Cl or NaCl
c. CHCl3 or CCl4
d. C2 H5 OH or H3 C − O − CH3
2. What are the optimal pressure (high or low) and temperature (high or low) conditions for dissolving a gas in water?
a. pressure
b. temperature
B. (10 points) Determine whether the statements given below are true or false. Write Y
if the statement is true, and N if the statement is false in the blanks provided. Do not
use T or F. You should not need to do any calculations for this part.
Consider 3 beakers. Each beaker has a solution made up of 1 mol of solute in 100 g
of solvent. Each of the solutions has a different density.
– Beaker 1 has C2 H5 OH (ethyl alcohol) in water
– Beaker 2 has C12 H22 O11 (sugar) in water
– Beaker 3 has NaCl in water
1. All the solutions have the same molality.
2. All the solutions have the same molarity.
3. Each of the three solutions has a different mass percent of solute.
4. The solutions in Beaker 1 and Beaker 3 have the same boiling point.
5. The solution in Beaker 2 has the lowest freezing point.
C. (4 points) What is the boiling point of a 0.33 m solution of benzoic acid in phenol?
Pure phenol boils at 182.0◦ C and has a boiling point constant of 3.56◦ C/m.
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Chem 128, Exam I
February 21, 2003
Name
D. (6 points) An aqueous solution of urea, CO(NH2 )2 (M = 60.06 g/mol) is prepared by
dissolving 20.03 g of urea in 456 g of water.
1. What is the mass percent of urea in the solution?
2. What is the molality of the solution.
E. (4 points) Consider a 0.200 m solution of a solute A in water. The density of the
solution is 1.12 g/mL. What is the molarity of the solution?
F. (5 points) What is the osmotic pressure of 0.00415 M solution of ammonium sulfate?
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Chem 128, Exam I
February 21, 2003
Name
II. (42 points)
A. (10 points) Consider the reaction
2X + Y → Z
∆H < 0
The rate law for the reaction is
rate = k [X] [Y]2
Answer the following questions in the blanks provided.
a. What is the order of the reaction with respect to Y?
b. What is the overall order of the reaction?
c. The rate constant for the reaction at 25◦ C is
1.0 L2 /mol2 -min. Would you expect the rate constant to
increase if the reaction is done at 35◦ C?
d. The rate constant is doubled when substance W is
added. W does not get used up by the reaction. What
is W called?
e. How does W affect the activation energy?
B. (15 points) Assume that a molecule ”A” reacts with two ”B” molecules in a one-step
process to give AB2 .
1. Write a balanced equation for the reaction.
2. What is the rate law for this reaction?
4. If the initial rate of formation of AB2 is 2.0 x 10−5 M/sec and the initial concentrations of A and B are 0.30 M, what is the value of the rate constant?
5. What are the units for k?
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Chem 128, Exam I
February 21, 2003
Name
C. (9 points) Consider the following first order decompostion of A at a certain temperature.
A → products
It takes 1.00 hour for 75% of A to decompose.
1. What is the half-life of A?
2. What is the rate constant for the decomposiion at that temperature?
3. What is the rate of decomposition at the same temperature when [A] is 0.500 M?
D. (8 points) Consider the following first order reaction at a certain temperature:
SO2 Cl2 (g) → SO2 (g) + Cl2 (g)
The half-life for the reaction at that temperature is 8.0 minutes. How long would it take
to reduce the concentration of SO2 Cl2 to 1,0% of the original?
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Chem 128, Exam I
February 21, 2003
Name
III. (42 points)
A. (15 points) Consider the exothermic reaction
2 NO (g) + Cl2 (g) 2 NOCl (g)
Which way will the equilibrium shift in response to the following changes? (Answer
right, left, more information needed or no effect.)
1. Increased volume?
2. Increased temperature?
3. Increased volume and decreased temperature?
4. NO is added and the temperature is raised?
5. A catalyst is added?
B. (8 points) Write a balance equation for the reaction of hydrogen gas with nitrogen
dioxide gas to form ammonia gas and steam. Use that equation to write an expression
for the equilibrium constant:
Equation:
Equilibrium constant expression:
C. (4 points) 3. At a certain temperature, the following reaction has an equilibrium constant of 64.0.
6 O2 (g) 4 O3 (g)
What is the value of the equilibrium constant for the reaction below?
2 O3 (g) 3 O2 (g)
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Chem 128, Exam I
February 21, 2003
Name
D. (15 points) Consider the following reaction at a certain temperature:
SO3 (g) + NO (g)
SO2 (g) + NO2 (g) 1. Calculate the equilibrium constant for the reaction at that temperature if at
equilibrium the partial pressures are PSO2 = PNO2 = 0.50 atm, and PNO = PSO3 =
2.0 atm.
2. Assume that at equilibrium one has the partial pressures of the species given
in Part(1). Then suppose enough SO2 (g) and NO2 (g) are added to raise both
their partial pressures temporarily to 2.00 atm. What are the partial pressures of
all species when equilibrium is re-established? To help you, you may fill in the
table below.
SO2
NO2
SO3
NO
Po
∆P
Peq
PSO2 =
PNO =
PNO2 =
PSO3
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Chem 128, Exam I
February 21, 2003
Name
IV. (25 points)
A. (15 points) A student dissolves 1.20 g of his unknown in 20.00 g of tertiary butyl
alcohol (TBA). Pure TBA has a freezing point of 24.5◦ C and a freezing point constant
of 8.0 ◦ C/m. The solution has a freezing point of 20.5◦ C.
1. What is the freezing point depression?
2. What is the molality of the solution?
3. How many moles of solute are there in the solution?
4. What is the molar mass of the solute?.
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Chem 128, Exam I
February 21, 2003
Name
B. (10 points) In a reaction involving the iodination of acetone, the following volumes
were used to make up the reaction mixture:
15.0 mL of 4.0 M acetone
15.0 mL of 1.0 M HCl
10.0 mL of 0.0050 M I2
10.0 mL of H2 O
It takes 100.0 seconds for the color of the iodine to disappear at 25◦ C.
1. What is the molarity of the acetone in the reaction mixture?
2. A second reaction mixture is made up where the concentration of I2 is halved.
All other concentrations are the same as in the original mixture. The following
volumes were used to make the third reaction mixture:
15.0 mL of 4.0 M acetone
15.0 mL of 1.0 M HCl
5.0 mL of 0.0050 M I2
15.0 mL of H2 O
If the reaction is zero order in I2 , how long will it take the I2 color to disappear at
25◦ C for the second reaction mixture?
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Chem 128, Exam I
February 21, 2003
Name
BONUS (15 points) All or nothing.
The bonus should be done only after you have completed the main part of this exam and
checked your work for errors. The time allotted for this exam does not include time for the
bonus. Trial and error solutions will not be accepted.
Consider 100-proof (50% ethyl alcohol by volume) Scotch whisky. If the temperature were
to drop to –10 C, could one still drink the Scotch? (i.e., Show by calculation that the
whiskey is a liquid.)
The density of ethyl alcohol (C2 H5 OH) is 0.79 g/ml.
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