INTRODUCTION TO COMPUTING AND EXCEL
Computing in the Sciences. Science is the systematic effort to understand the nature of the
physical universe. Scientists have developed powerful tools that are used in this enterprise;
the computer is one of these. Computing as a tool is introduced in general chemistry
because of its importance in the modern practice of science. An introduction to scientific
computing is appropriate as a component of your initiation to science and the scientific
method.
Scientists use computers in a variety of ways. Computers are required in the control of
sophisticated instrumentation and in the acquisition of data. For example, in a typical NMR
(nuclear magnetic resonance) experiment, a high-power transmitter is turned on and off in
ca. 3 s and 4,000 data points are collected in 0.5 s. Computing is invaluable in the
manipulation of the massive amount of data generated in the modern laboratory and in the
calculation of quantities from first principles. Chemists are now able to calculate the
structure of small molecules by solving the Schroedinger wave equation. The scientific
literature has grown to the point that several million research articles are published per year.
The immensity of the scientific literature was a major driving force behind the development
of on-line data bases. Finally, the communication of results discovered in the laboratory is
an essential component of the scientific method. Word processing has greatly assisted the
preparation of scientific papers and the World Wide Web provides direct access to useful
data and the international scientific community.
The scope of scientific computing covers the following major categories: control of
instrumentation, large-scale numerical calculations (number crunching), data analysis, and
communications. A scientist must be familiar with a variety of software as no single
software package deals with all of these applications. In general chemistry, we shall focus
on data analysis which includes most routine chemical calculations. Data analysis is the
most important application of computing in the day-to-day practice of science. Scientists
perform a wide range of fairly straightforward calculations; work in a research laboratory is
the polar extreme of the assembly line.
We have chosen a spreadsheet approach using Microsoft Excel to handle the computation in
the General Chemistry laboratory. This document serves as a guide to resources which will
enable you to make effective use of spreadsheets and Microsoft Excel. One develops facility
with computer applications with practice and this introduction includes a short exercise
which focuses on spreadsheet techniques which you will find useful.
How to Access Excel. Students who use the PC’s in Seaver North 113 or those in the
computer labs maintained by the computer center will use the Excel program loaded on each
PC. Those of you who wish to use your own PC if you have a license for Excel. Please note
that portions of this and following exercises will require the Data Analysis add-in which
does not come with the standard package. Excel installed on the PC's in the classrooms does
contain the extra options.
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You need to log on to the campus network in order to print and to access your user space. If
you are using a computer in Seaver North room 113, click on the Excel icon on the desktop.
The following are a few other things to keep in mind when using network applications:
1. Remember to save your work to a floppy disk or preferably to your user space
before exiting the program.
2. Remember to log off when you're finished otherwise the next user will have
access to your user space.
Introduction to Excel. The following example is designed to familiarize you with entering
and manipulating data in Excel spreadsheets and will be presented in lab during the Excel
demo on the first day of lab. The problem deals with the analysis of a crude sample of
sodium carbonate isolated from a dry lake deposit. The sample is dissolved in acid-free
water and is titrated with 0.1098 M hydrochloric acid to the equivalence point. Two protons
react with each carbonate anion. The following data were found in a notebook.
sample #
1
2
3
4
5
6
sample mass (g)
0.1400
0.1511
0.1307
0.1612
0.1488
0.1534
volume of HCl (ml)
20.42
22.13
19.09
23.49
22.12
47.22
1) Start Excel and enter the label “mass(g)” at the top of the first column, i.e. in the A1
cell. Enter the 6 values of the sample mass in the cells A2 through A7.
2) Similarly label the second column “V(ml)” and enter the volumetric data.
3) Label the third cell “n H+” and calculate the moles of protons for the first sample by
entering the following in the C2 cell: =0.001*B2*0.1098. The result will appear in the
C2 cell. If the attempt to perform the calculation fails to yield a numerical result, the
following incantation should exorcise the Excel demons. Clear the offending cell and
click on an empty cell in the spreadsheet. If you try again using proper syntax, the
procedure should work.
4) Now automatically complete the calculations for the remaining cells (cells C3-C7) by
completing these steps: click on the C2 cell, click on the Copy item in the Edit menu,
select the cells C3-C7, and click on the Paste item in the Edit menu.
5) Label the fourth cell "n CO3" and calculate the moles of carbonate for the first sample by
entering the following in the D2 cell: =C2/2. Complete cells D3 through D7 as you did
in step 4.
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6) Similarly label the fifth column “g CO3(g)” and calculate the mass of sodium carbonate
in each sample. The MW of sodium carbonate is 105.99 g/mole. The sequence is
calculate in the E2 cell, copy, select, and paste.
7) Finally calculate the wgt-% of sodium carbonate in the sixth column.
8) The volume for the sixth run is clearly incorrect. It turns out to be a typo and the correct
value is 22.42 ml. Enter the correct number. Note that the spreadsheet is automatically
updated and the corrected results appear in cells C7-F7.
9) Now use the Excel functions to calculate the average, standard deviation, standard
deviation of the mean, and 95% confidence interval of the mean.
a) Suppose we wish to place the average of the results in the F9 cell. First click on
the F8 cell and enter a label, e.g. avg 1-6.
b) Next click on the empty F9 cell, designating it to receive the result from the
following steps.
c) Next click on the following:
Tools
Data Analysis
Descriptive Statistics
OK
d) A Descriptive Statistics window will open up. Select the following output
options: Output Range, Summary Statistics, and Confidence Level for Mean.
The default level is 95%.
e) Click in the box beside Output Range. Then click in the cell where you wish the
report to begin, i.e. its upper left hand corner.
f) Click in the box beside the field Input Range. Then using your mouse, select all
values of the wgt-% sodium carbonate in the F column of your spreadsheet and
finally on OK in the Descriptive Statistics window. A report of the desired
statistics will appear in your spreadsheet. The item labeled standard error is the
standard deviation of the mean. Note that this utility was written so that you
must select a contiguous range of values.
g) An alternate approach is required if the data that you wish to select are not
contiguous. You may want to do this if you suspect that one datum is flawed. In
our example, notice that the result for the fifth run deviates significantly from the
average. Perhaps the result is in error. You could copy the data in your
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worksheet, i,e. duplicate the cells in another section of the worksheet, and then
delete the row with the suspected bad datum.
h) Alternatively you can take another route which requires a separate pass for each
statistic. First click on the cell to hold the result. Click on the following:
Insert
Function
i) An Insert Function box will appear. Select the desired statistic, e.g. AVERAGE,
from the list of options and then OK.
j) A Function Argument box will now appear. Select the data but not the datum for
the fifth run. To choose cells selectively, hold down the Control (Ctrl) key before
you click on each cell you wish to include. Click on OK when the data have been
selected.
k) Similarly calculate the standard deviation and observe that the standard deviation
improves significantly with the removal of the suspect datum.
l) Finally use spreadsheet techniques to calculate the standard deviation of the mean
and the 95% confidence interval of the mean.
10) Save the spreadsheet on your user space or to a diskette by clicking on the Save As item
in the File menu. When the Save window appears, select the appropriate drive and
provide a unique file name (the extension is .xls) before clicking OK. Get in the habit of
saving any work that is important (Save early, Save often!). Do not save your work on
the hard drive of lab computers. They are frequently cleaned of extraneous files in order
to maintain space for software.
11) Confirm that you have saved something. Close the present spreadsheet by clicking on
Close in the File Menu. Then read back in the spreadsheet that you just saved by
clicking on Open in the File menu. Select the drive and file name and click OK.
12) Print out your completed spreadsheet. Click on the Print item in the File menu. When
the Print window appears, select the printer.
13) Finally exit from Excel by clicking on Exit in the File menu. Exit from Windows and
log off of the Novell network.
excel.doc, 3 July 1997, WES
updated January 2, 2002, JMT; 8 August 2004, WES
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