Name ________________________
CHEM 1471 (Tuesday)
Fall 2010
Buckley
Show your work on all numerical problems to receive full credit.
Experiment #4: Rates of a Chemical Reaction: A Clock Reaction
1. (10 points) In conducting this experiment, you mixed together two solutions and timed
the appearance of a dark blue color. If the following two solutions are mixed together,
give the molar starting concentrations of the I- and the S2O82-.
Solution 1
4.0 mL of 0.150 M KI
6.0 mL of 0.250 M KNO3
5 mL of 0.005 M Na2S2O3
4 drops of starch solution
Solution 2
2.5 mL of 0.100 M K2S2O8
7.5 mL of 0.100 M (NH4)2SO4
2. (10 points) In the course of this experiment you found the rate law for the reaction of Iwith the peroxydisulfate ion, S2O82-, is:
Rate = k [I-][S2O82-]
If one your reaction mixtures took 58 s to turn blue, how long would it take for a reaction
mixture with one-half the starting [I-] and one-half of the starting [S2O82-] to turn blue?
Explain your reasoning.
Experiment 7/8: Determining the pKa of a Weak Acid
3. (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 29.45-mL of your
NaOH to reach the end point during the titration. What is the molarity of your NaOH
solution? (Show your work.)
4. (10 points) In the titration of an unknown monoprotic acid with your NaOH solution
from above, 0.2473-g of the monoprotic acid required 31.25-mL of the NaOH solution to
reach the endpoint. What is the molar mass of the unknown monoprotic acid?
5. (10 points) In this experiment, explain how you determined the pKa of your weak acid
from the pH vs. volume of NaOH plot.
The Determination of an Equilibrium Constant:
In both this experiment and the phenolphthalein experiment you became familiar with Beer’s
Law which gives the relationship between the absorbance of a solution and its concentration.
6. (10 points) Suppose one collected the data below in an attempt to determine the Beer’s
Law plot for absorbance as a function of FeNCS2+ concentration.
[FeNCS] Absorbance
0.00004
0.152
0.00008
0.307
0.00024
0.91
0.00032
1.23
Construct a Beer’s Law plot on the grid below for these data.
Grid for Making Plot - Problem 6
1.4
Absorbance
1.2
1
0.8
0.6
0.4
0.2
0
0
0.00005
0.0001
0.00015
0.0002
Concentration
0.00025
0.0003
0.00035
7. (5 points) From your plot above, what is the approximate slope and intercept of the best
line connecting the points?
8. (5 points) Based on your answer to problem 7, what would the concentration of an
FeNCS2+ solution be if its absorbance was 0.700?
Freezing Point Depression: Determination of the Molar Mass of an Unknown Substance
9. (10 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 measured amount of dissolved unknown was again
determined to find the freezing point depression. The overall relationship was:
ΔTf = nmKf
Answer the following questions.
a. 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.
b. Suppose you calculate the molar mass of the unknown assuming the solute did not
break into other fragments in solution. Would your calculated molar mass of the
unknown be higher than, lower than, or equal to the actual molar mass if the
solute actually did break into other fragments? Explain.