Download Answers PRACTICE EXAM II Spring 2008 Part I. Multiple Choice (3

Chemistry 122 (Tyvoll)
PRACTICE EXAM II
Answers
Spring 2008
Part I. Multiple Choice (3 points each -- Total points = 45)
1. Which of the following hypothetical steps is predicted to be exothermic?
3. separated solvent and solute molecules → solution.
2. Calculate the molality of methanol in a solution prepared by dissolving 59.3 g methanol,
CH3OH, (32.04 g/mol) in 150 g ethanol.
5. 12.3 m
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3. Consider the reaction A ' 2 B, where the value of KC is 1.4 × 10 . Which statement about the
system at equilibrium is correct?
4. The amount of A is much less than the amount of B.
4. Consider the equilibrium reaction, N2 (g) + 3 F2 (g) ' 2 NF3 (g)
In a particular experiment, the following concentrations are found at equilibrium: [N2] is
0.048 M, [F2] is 0.093 M, and [NF3] is 6.55 × 10-5 M. What is the value of the equilibrium
constant?
3. 1.1 × 10-4
5. What is the mole fraction of biphenyl, C12H10 (154.2 g/mol), in a 0.555 m C12H10 solution in
benzene, C6H6 (78.1 g/mol)?
1. 0.0416
6. At 1123 K, a dynamic equilibrium exists between carbon monoxide, carbon dioxide and solid
carbon:
ΔH = + 172.5 kJ
C (s) + CO2 (g) ' 2 CO (g)
If KC = 0.153, what is the value of Kp for this reaction?
5. Kp = 14.1
7. Predict which one of the following pairs of compounds would form a solution.
4. Solute: CH3(CH2)8CH2OH
Solvent: H2O
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8. Which one of the following best describes what occurs in a reaction system when it reaches a state
of dynamic equilibrium?
4. the rates for both forward and reverse reaction processes are the same
9. Which of the following solutions has the lowest freezing point?
3. 1.0 m Na2SO4 in water
10. Identify the acids/bases on respective sides of the equation: HCO31- + OH1- ' CO32- + H2O
4. acid + base ' base + acid
11. Which of the following is not true in a dynamic equilibrium describing a saturated solution?
5. Rate of dissolving is double the rate of crystallization.
12. The greatest gas solubility in water is predicted under what conditions?
4. low T, high P
13. The conjugate acid of H2PO4− is
4. H3PO4
14. Which one of the following is the weakest acid?
1. HF
15. What is the total number of moles of ions in 0.250 L of 0.391 M Na2CO3, a strong
electrolyte?
3. 0.293 mol
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Part II - Total points = 55. Answer each of the following questions. SHOW ALL WORK !
1. (10 points) Osmotic pressure measurements are routinely used in the determination of
molecular weights for high molecular proteins. An African swine fever virus-induced protein
(p12) was studied in virus-infected Vero cells using the monoclonal antibody 18B.B11. If an
aqueous solution containing 539 mg of p12 protein per 100 mL at 25 0C has an osmotic
pressure of 8.35 mmHg, calculate the molar mass of the p12 protein in g/mol and in kDa if
1 Dalton (Da) = 1 g/mol. 1 atm = 760 mmHg
You must show all work for credit or partial credit!
M = 12.0 kDa (12,000 g/mol)
2. (10 points) At high temperatures, like those found in internal combustion automobile engines,
N2 reacts with O2 to produce NO (g), a primary air pollutant. Consider this reaction, at 2300 K
where,
N2 (g) + O2 (g) ' 2 NO (g)
KC = 1.7 x 10-3
If 0.300 mol NO (g) are placed into a 20.0 L reaction vessel and heated to 2300 K, calculate
the equilibrium concentrations of all three species (N2, O2 and NO). Show all work,
including the appropriate ICE table, for credit!
[N2] = [O2] = 0.00735 M
(from quadratic)
[NO] = 0.00030 M
Is this reaction reactant-favored or product-favored at 2300 K? Explain briefly.
Reactant-favored (KC << 1)
3. (10 points) When 1.50 g of a newly synthesized, molecular, nonelectrolyte compound were
dissolved in exactly 75.0 g of hexane, C6H14, the freezing point of the solution was lowered from
6.50 0C (pure hexane) to 2.70 0C for the solution. For hexane, Kf = 20.2 0C/molal.
(a) Calculate the molality of the solution. Show all work for partial credit !
m = 0.188 m
(b) Calculate the molar mass of the solute. Show all work for partial credit !
M = 106.3 g/mol
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4. (10 points) Consider the reaction in which 2 NH3 (g) ' N2 (g) + 3 H2 (g), where
ΔH = + 92.2.28 kJ at 298 K. If N2 (g), H2 (g) and NH3 (g) are initially in equilibrium in a
reaction vessel, how will each of the following changes affect the equilibrium concentrations
shown below? Assume constant volume except for the last case. Answer the question by
writing I (increase), D (decrease) or NC (no change) in each of the blanks provided.
Change
[NH3]
[H2]
KC
Some H2 is removed from the container
__D__
__D __
__NC__
The temperature is increased
__D__
__I___
__I___
The pressure of the NH3 is decreased
__D__
__D___
__NC__
The volume of the container is increased
__D__
__I ___
__NC__
5. (10 points) Hydrogen gas can be generated at room temperature (298 K) in a reaction between
methane gas and water vapor: CH4 (g) + H2O (g) ' CO (g) + 3 H2 (g), where KC = 0.94.
If the initial concentrations of the gaseous species are [CH4] = 0.100 M, [H2O] = 0.100 M,
[CO] = 0.0500 M and [H2] = 0.200 M, will more H2 be formed, will it decompose to make
more CH4 and H2O once the mixture achieves equilibrium at 2300 K, or is the system already
at equilibrium?
Make the appropriate calculations and then clearly discuss your results for credit on this
question.
QC = 0.0400 << 0.94 = KC
Since QC << KC, rxn will shift to the right.
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6. (5 points) According one text, a 0.92% (w/v) NaCl (0.16 M) solution is “isotonic” with the
fluid in red blood cells. Another solution used, “D5W”, is 5.5% (w/v) solution (0.31 M) of
glucose in water and is also isotonic with the fluid in red blood cells. Explain how these
solutions can have such different concentrations and still both be isotonic with the fluid in red
blood cells. Also explain what might happen if the concentration of NaCl in such a saline
solution that was in contact with red blood cells was less than 0.92%.
You must be complete and specific to receive credit on these questions!
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