HINTS FOR 124 PRELABS
PURPOSE
To be kind.
Experiment 2 – Intermolecular Forces
There no hints worth sharing.
moles of each. Next use the information
(molar mass, density, molarity) given in
the Reagents Provided section of the
experiment to relate the calculated
number of moles to mL or g, as needed.
1. (a) A simple dilution calculation.
(b) Use Beer’s law.
2. Use Graphical Analysis (GA) exactly as
instructed in steps 18-20 of the experimental
procedure. The only differences are:
 You need to manually enter your
time and absorbance data.
Be
careful; a typo in one of the numbers
can make your results incorrect. So,
it is a good idea to leave GA open,
without deleting your data, until after
you check your answers in Prelab.
 Unless you manually change the
column headings for the data you
enter, the column of absorbances
will be labeled Y. So, the equation
you enter in step 11(c) will be
“Y”/xxx where xxx is the answer to
prelab question 1(b).
3. No new calculation should be required
here. Simply use the slope of your line
as discussed in the Background section
of the experiment.
4. (a) Based on the appropriate linear
equation, the initial concentration can be
determined from the y-intercept as
follows:
First-order: ey-int
Second-order: 1/y-int
(b) Refer to equation (5) or (6) of the
Background section, as appropriate.
Experiment 3 – Analysis of Aluminum
2. Instructions for using Graphical Analysis
to generate graphs and perform linear
regression are provided on page xv of
the coursepack.
4. This should be a simple dilution
calculation.
Experiment 4 - Electrolysis
0. This is a dilution calculation.
1. (a) current x time = charge.
3. (b) First determine the pressure in mm
Hg using the formula provided. Then
convert it into atm when entering the
answer into PRELAB. (760 mm Hg = 1
atm.)
(c) Be sure all variables are entered
using the correct units (same units used
in R = 0.0821 L atm mol-1 K-1).
5. The
formula
used
to
calculate
‘electrochemical’ percent yield is given in
Data Analysis question 5 at the end of
the experiment.
Experiment 5 – Solvation and Freezing Point
The necessary background and equations
are in the introductory sections of the
experiment. Therefore, there are no hints
for this prelab exercise.
Experiment 6 – Kinetics
0. In step 14 of the experimental procedure
it tells you the volume and molarity for
each solute. Use these to calculate the
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Hints for Prelabs
Experiment 7 – Iron(III)-Thiocyanate Reaction
2. (c) Note that the moles of product
obtained is specified in the opening
paragraph of the prelab.
3. (a) & (b) In this question you are NOT
assuming the reaction went to
completion. So, use the actual moles of
product formed (as specified in the
opening paragraph) to calculate how
many moles of each reactant have been
consumed (using the appropriate
stoichiometric ratios). Moles consumed
can be subtracted from the initial
number of moles present (calculated in
question 1) to get the moles left
unreacted.
(c) The moles of AB2 formed (which is
needed to get its molarity) is simply the
number given in the opening paragraph
(do NOT use the answer to 2(b)). Try to
avoid rounding error by not truncating
your molarities when you calculate them.
Experiment 10 – pKa and Molar Mass
1 & 2. The actual numerical answers to the
questions are given in the text
accompanying
the
referenced
experimental figures (so you do not
need to estimate them off of the graph).
Experiment 11 – Electrochemical Cells
This prelab is newly revised to go with the
current version of the experiment. The
biggest potential cause for frustration is
if you get incorrect vales for the required
reduction potentials. Most web sources
are accurate, but not all. Therefore, we
strongly recommend looking in your
textbook to get the proper values.
Because we allow for the reduction
potentials you use to slightly differ from
those in the textbook, this prelab is way
less fussy about rounding and significant
figures.
2. This is just simple subtraction. Do not
overthink it.
3. Using 96,500 for F will give the free
energy in J. PRELAB will want you to
enter the answer in kJ (which is typically
used for free energy), so use 96.5 for F.
The hardest part is getting the correct
value for n (which depends upon the
specific reaction). You need to use the
complete balanced reaction to see how
many electrons are transferred each
time the complete reaction occurs.
(Hint: The correct value of n is NOT 3.)
See the background section of the
experiment, your textbook, or your
instructor for help, as needed.
Experiment 8 – Qualitative Analysis
The only possible answers, when asked for
a chemical, are those species listed at
the top of the prelab questions.
Experiment 9 – pH Meters
5. A sample buffer capacity calculation is
show in the background information in
the experiment.
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Hints for Prelabs
Experiment 6 – Kinetics: The Sequel
1. Add the volumes of all of the reagents
used (in a single row) to get the final
volume for your dilution calculation. This
final volume should be the same in all of
the solutions.
2. The exact same hints apply as for
question 2 of the previous experiment.
3. Remember when you are told to graph
alpha vs. beta that alpha is on the y-axis.
(b) The rate constant can be determined
from the slope as described in the
background section entitled Determining
the Order.
pKa help for when standardization is added
P2. Calculate the moles of NaOH, which will
also equal the moles of acid. Then
convert moles of acid to grams using the
given molar mass.
[Remember that KHP is shorthand
notation
for
potassium
hydrogen
phthalate, so don’t try to determine its
molar mass by adding up K + H + P.]
1. Again, use the given molar mass of
KHP.
Experiment 7 – Analysis of Bleach
2. The moles of thiosulfate simply equals
the moles of sodium thiosulfate. From
the moles of thiosulfate the moles of I2
can be easily determined from reaction
(2). Now use reaction (1) to get the
moles of hypochlorite, ClO-.
3. The moles of sodium hypochlorite is
simply the same as the moles of
hypochlorite.
4. Look at the hint provided in question 4 of
the Data Analysis section of the
experiment. The density to use is at the
top of your prelab questions.
5. Percent by mass is the same thing as
weight percent.
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