Name ________________________
CHEM 1471 (Thursday)
Final Exam
Fall 2010
Buckley
Show your work on all numerical problems to receive full credit.
The Determination of an Equilibrium Constant
1. (10 points) In conducting this experiment, you mixed together two solutions and
measured the absorbance of the resulting equilibrium mixture. What is the initial
concentration of Fe3+ and of SCN- if the following solutions are mixed together in a 25mL volumetric flask?
0.002 M NaSCN
(in 0.25 M HNO3)
3 mL
0.002 M Fe(NO3)3
(in 0.25 M HNO3)
5 mL
0.25 M HNO3
dilute to mark
2. (10 points) The equilibrium being studied here is:
Fe3 (aq)  SCN  (aq)
FeSCN 2 (aq)
If the solution mixed together in Problem 1 produced a concentration of FeSCN2+ of
0.00020 M, what is the value of the equilibrium constant?
3. (5 points) Whose Law did we take advantage of to find the equilibrium concentration of
FeSCN2+ in this experiment?
Experiment 7/8: Determining the pKa of a Weak Acid
4. (10 points) In this experiment, you first made up a solution of NaOH to be used for
titration. Suppose 0.5135 g of KHP (molar mass = 204.2) requires 34.25-mL of your
NaOH to reach the end point during the titration. What is the molarity of your NaOH
solution? (Show your work.)
5. (10 points) In the titration of an unknown monoprotic acid with your NaOH solution
from above, 0.2473-g of the monoprotic acid required 28.25-mL of the NaOH solution to
reach the endpoint. What is the molar mass of the unknown monoprotic acid?
6. (10 points) In this experiment, explain how you determined the pKa of your weak acid
from the pH vs. volume of NaOH plot.
Experiment 4: Rates of a Chemical Reaction – A Clock Reaction
7. (10 points) Suppose one collected the data below in an attempt to determine the order of
this reaction with respect to the concentration of [I-].
log(rate)
-5.68
-5.43
-5.11
-4.92
log[I]
-1.70
-1.40
-1.10
-0.92
Construct a plot of log(rate) vs. log[I] on the grid supplied below.
Grid for Making Plot - Problem 7
-4.80
-4.90
-5.00
log(rate)
-5.10
-5.20
-5.30
-5.40
-5.50
-5.60
-5.70
-5.80
-1.80
-1.60
-1.40
-1.20
-1.00
-0.80
-0.60
-0.40
-0.20
0.00
log[I]
8. (5 points) From your plot above, what is the approximate slope of the best line
connecting the points? (Note: you can use the edge of one of the other test pages as a
straight edge if you want.)
9. (5 points) Using an intercept of -4.04 and your slope from Problem 8, what is the log[I]
that gives a log(rate) of -4.60?
Freezing Point Depression: Determination of the Molar Mass of an Unknown Substance
10. (5 points) In this experiment you determined the molar mass of an unknown substance
by determining the magnitude of the freezing point depression it caused in t-butyl
alcohol. As you may recall, the freezing point of pure t-butyl alcohol was determined
and then the freezing point with a measured amount of dissolved unknown was again
determined to find the freezing point depression. The overall relationship was:
ΔTf = nmKf
Answer the following questions.
If the unknown broke into multiple fragments in solution, would the freezing point
observed be higher than, lower than, or the same as if the solute stayed intact? Explain.