HINTS FOR 122 PRELABS
PURPOSE
To be kind.
Experiment 1 – Determination of Density
The biggest problem is the number of
significant figures. In all of the problems the
correct answer should have four sig figs, not
three, as is so often the case.
3. This is a simpler question that some
students make it. Your answer should
be a simple whole number that equals
the number of ions that are created
when one formula unit of the compound
dissociates in solution.
Experiment 2 – Fun with Gas Laws
There is no prelab for this experiment.
Experiment 6 – Titration of an Unknown Acid
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.
Experiment 3 – Density of Aqueous NaCl
Questions 1-3 could all be easily answered
using a hand calculator.
However, to
answer question 4 correctly you almost
certainly need to use Graphical Analysis.
We’re not kidding or trying to trick you. If
you do the calculations by hand, you need to
keep lots of sig figs to avoid rounding error
when you generate the graph. So, it is much
easier to just use Graphical Analysis to do
the whole thing. We use this program a lot,
so it will be easily worth the time it takes for
you to learn to use it.
4. The best way to solve for the mass % is
to use the y=mx+b formula where y is
the unknown density and x is the mass
%. m and b come from your graph.
Experiment 7 – Solution Calorimetry
3. The simple equation to use is the first
one in the experiment. Be sure to
convert J to kJ.
2. Since equal volumes were used, the
smaller moles will result from the
reactant with the smaller concentration.
Further, since it is a 1:1 stoichiometric
ratio, the one with the fewer number of
moles will be the limiting reactant.
Experiment 8 – 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 4 – Synthesis of Zinc Iodide
1. Don’t use the molar mass of Cl2 or P4
when converting grams to moles. It
specifies atoms (not molecules), so just
use the molar mass of Cl or P.
Experiment 5 – Conductivity
1. The order of magnitude of the
conductivities should be sufficient to
identify the type of electrolyte.
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Hints for Prelabs
experiment. The density to use is at the
top of your prelab questions.
5. Percent by mass is the same thing as
weight percent.
Experiment 9 – Analysis of Calcium
2. Use the answer to question 1 and mass
of the tablet given in the opening
paragraph.
3. Another way to phrase the question
would be: What percentage of the molar
mass of CaCO3 is due to the Ca?
Experiment 10 – Analysis of Phosphorus
2. You need to look at both reaction (1) and
reaction (2). Your answer should be a
simple whole number.
5. Here you need to use the molar mass of
P4, not the molar mass of P.
6. Be sure to convert into a percentage (by
multiplying the fraction by 100%).
Experiment 11 –Spectrometry
P1. Table I, at the beginning of the
experiment, will help you answer this
one.
1. This is a dilution calculation.
3. Use y = mx + b to solve for the
concentration (x) from the absorbance
(y) and the m and b values obtained in
question 2.
Experiment 12 – Analysis of Aspirin
1. Each question is as simple, or simpler
than it looks. Students always make this
prelab a lot harder than it is. A couple of
the questions don’t even require a
calculator.
Each question uses the
answer to the preceding question.
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