Page 49 - Modern Analytical Chemistry

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32 Modern Analytical Chemistry

SOLUTION
Substituting known volumes (with significant figures appropriate for pipets
and volumetric flasks) into equation 2.4

2+
(ppm Cu ) o ´20.00 mL =4.62 ppm ´50.00 mL
2+
and solving for (ppm Cu ) o gives the original solution concentration as 11.55
2+
ppm. To calculate the grams of Cu we multiply this concentration by the total
volume
11 55 mg Cu 2+ 1 g –3 2+
.
.
´ 250 0 mL ´ = 2.888 ´ 10 g Cu
6
mL 10 mg
The weight percent Cu is then given by
–3
2.888 ´ 10 g Cu 2+
´100 =0 231% /ww Cu
.
1.25 g sample

2F The Laboratory Notebook

Finally, we cannot end a chapter on the basic tools of analytical chemistry without
mentioning the laboratory notebook. Your laboratory notebook is your most im-
portant tool when working in the lab, providing a complete record of all your work.
If kept properly, you should be able to look back at your laboratory notebook sev-
eral years from now and reconstruct the experiments on which you worked.
Your instructor will probably provide you with detailed instructions on how he
or she wants you to maintain your notebook. Of course, you should expect to bring
your notebook to the lab. Everything you do, measure, or observe while working in
the lab should be recorded in your notebook as it takes place. Preparing data tables
to organize your data will help ensure that you record the data you need and that
you can find the data when it is time to calculate and analyze your results. Writing a
narrative to accompany your data will help you remember what you did, why you
did it, and why you thought it was significant. Reserve space for your calculations,
for analyzing your data, and for interpreting your results. Take your notebook with
you when you do research in the library.
Maintaining a laboratory notebook may seem like a great deal of effort, but if
you do it well you have a permanent record of your work. Scientists working in aca-
demic, industrial, and governmental research labs rely on their notebooks to pro-
vide a written record of their work. Questions about research carried out at some
time in the past can be answered by finding the appropriate pages in the laboratory
notebook. A laboratory notebook is also a legal document that helps establish
patent rights and proof of discovery.

2G KEY TERMS

balance (p. 25) dilution (p. 31) formula weight (p. 17)
concentration (p. 15) equivalent (p. 17) meniscus (p. 29)
desiccant (p. 29) equivalent weight (p. 17) molality (p. 18)
desiccator (p. 29) formality (p. 15) molarity (p. 15)