Graphs for Exp 1 - addendum

[robert zellmer]

Someone asked about where the axes should start for the graphs for exp 1.

Since the volumes (and masses) from the beaker are different than those
for the other glassware the graph would have a lot of empty space if you
include the origin.  It is not necessary to include the origin on the 
graph,
for any of the graphs.  In order to have essentially the same scaling 
for your
data points on all four graphs you can do one of the following:

1) Have the graphs for the buret, pipet and grad. cyl. start at the origin.
      Since these graphs start around 5 mL  (and around 5 g) perhaps and
      the first data point is about 5 units from the origin you can 
start the graph
      for the beaker at around 15 mL and a mass of around 15 g or so. For
      visual purposes this will give the graphs about the same scaling 
on the axes.
      You do have the R^2 values to look at but it's nice to be able to 
visualize
      how well the points fit the best-fit line.

2)  You could start each graph about 1 unit away from your first data 
point.
      This means for the buret, pipet and grad. cyl. your graphs would 
start
       at about 4 mL and 4 g.  The one for the beaker would start at 
about 19 mL
      and 19 g.  Of course this all depends on exactly where the first 
data point
      winds up.

      This latter method would allow the points to spread out more. It 
wouldn't show
      the origin but you don't need it as the eqn. gives you the 
y-intercept for your
      data.  You need this intercept for your discussion.

Whenever possible, it's a good idea to spread out your data as much as 
possible
and that may mean not starting at the origin (the 0,0 point).  This 
allows you to see
if you perhaps have a "bad" data point, which perhaps shouldn't actually 
be included
in determining the best-fit line or in further calculations.

3) A couple of people have asked whether the lines on the graphs should be
forced through the origins.  We understand they should go through the 
origin.
However, you shouldn't force them through the origin (this can be done 
in Excel
and you will do it for one of the graphs later in the semester). The 
fact the
y-intercept doesn't go through the origin tells you something (one type 
of error
you may have can cause it to not pass through the origin like it should).

Dr. Zellmer
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