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for (hopefully) ease of understanding. If you have any questions, comments, or
suggestions, please do not hesitate to call (298-3399 x-5658) or email
([email protected]) or stop by (G-12A) to see me.
Determining Data Ranges:
One of the most common mistakes made is choosing a scale that utilizes only a
very small corner of the graph paper rather than the entire piece. This results in graphs
that can be difficult to see and larger than necessary experimental error should i be
necessary to find the slope, intercept and/or use in predictions. Therefore, it is important
to be able to choose a scale for the graph that is appropriate.
To begin, we have to decide what kind of graph we are making. There are three
general categories; (1) graphs to cover only the experimental points; (2) graphs that must
extend to some smaller number well beyond the smallest experimental point (often zero);
and (3) graphs that must extend to some greater number well beyond the largest
experimental point. Regardless of the type of graph that one must make, begin by
examining your data. You’ll have a set of data that includes both “x” and “y” data.
Generally, “x” is taken to be exact, while “y” is the measured quantity, that is, the
quantity that has experimental error. For instance, suppose I were to make a graph of
boiling points versus molecular weight. You’ll find a table of such data below in the
appendix. The boiling points I measure experimentally. This implies that there could
(and will) be some experimental error associated with this measurement. The molecular
weight, however, can be easily calculated with very little error (much much smaller error
than the boiling point). Therefore, I make my measurement with the greatest error
(boiling point) the “y” axis, and the measurement with the smallest error (molecular
weight) the “x” axis.
For both the x and y axis, determine your largest and smallest value. If your
graph must extend to some range other than that covered by the experimental data, use
these as your limits instead. For instance, perhaps my graph of boiling points versus
molecular weight must cover very small molecular weights, say down to the molecular
weight of methane CH4 , which has a molecular weight of 16 g/mol. However, my Dakota State University page 214 of 232 Plotting Experimental Data General Chemistry I and II Lab Manual experimental data has molecular weights ranging from 86 g/mol to 142 g/mol. Then I
will make a plot that runs from at most 16 g/mol through at least 142 g/mol. For ease of
calculation, I may choose numbers close to our limits, such as, say, 10 g/mol to 150
g/mol as my limits. Doing this tends to make it easier to figure out the scale.
As for the y axis, it is a little more difficult. We want to choose a minimum value
that we expect will be low enough include the boiling point of a chemical with molecular
weight of 10 g/mol. It would be much easier if we knew we wanted the graph to go to
some finite amount. Lo...
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