Graphs for Exp 1 - addendum

[Zellmer, Robert]

I've received questions about where the axes should start for the graphs for
exp 1 and whether the lines should be forced through the origin.

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) If your data was perfect the lines should go through the origin, since
    if the mass is zero the volume should be zero and vice versa. 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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