1
Chemistry 222
Name__________________________________________
Spring 2008
Exam 1: Chapters 1-4
80 Points
Complete two (2) of problems 1-3 and four (4) of problems 4-8.
CLEARLY mark the problem you
do not want graded.
You must show your work to receive credit for problems requiring math.
Report your answers with the appropriate number of significant figures.
Do two
of problems 1-3. Clearly mark the problem you do not want graded.
(10 pts each)
1.
You need to prepare a 50.0 mL of solution that is 100.0 ppm magnesium.
Clearly describe
how you would prepare this solution starting from the points below.
a. starting with solid magnesium chloride
Remember, magnesium chloride is MgCl
2
(FW = 95.211 g/mol)
100 mg
Mg
2+
x 1 mol Mg
2+
x 1 mol MgCl
2
x95.211g
MgCl
2
x
1 g
x0.0500 L
=
0.0195
8
g MgCl
2
1 L
24.305 g
1 mol Mg
2
+
1 mol MgCl
2
1000 mg
So, dissolve 0.0196 g MgCl
2
in 50.0 mL of solution
b.
starting with a 0.100 M magnesium chloride solution
Since each mole of MgCl
2
that dissociates liberates 1 mole of Mg
2+
, a 0.100 M MgCl
2
solution
is also 0.100 M Mg
2+
100 mg
Mg
2+
x 1 mol Mg
2+
x0.050 L
x
1 L
=
2.06 mL
1 L
24.305 g
0.100 mol Mg
2+
So, dilute 2.06 mL of 0.100 M MgCl
2
solution to 1.00 L
2.
We ignore the contribution of buoyancy in virtually all of the mass measurements we make in
the laboratory.
How can we get away with this?
Identify one situation where we would be
unable to ignore buoyancy-introduced error.
The buoyancy correction accounts for the varying volume of air displaced when a sample is
weighed compared to the volume displaced when the balance was calibrated with calibration
weights.
When the density of the sample being weighed is similar to the density of the
balance weights (8 g/mol), the error due to buoyancy is minimal (remember the plot we
discussed in class).
In general buoyancy errors are minimal because we have been
weighing solid samples and because we do our critical weighing
by difference
.
If we were to
weigh samples of very low density (like water or organic solvents or especially gases), we
should account for buoyancy errors.

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