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I've had a few people come ask me about the graph and calculating <br>
the slope for the pre-lab for BAR. There seems to be a <br>
misunderstanding of the purpose of a best-fit line and how to do the
<br>
slope. Several people took data points for the slope instead of
points <br>
from the line. <br>
<br>
So I guess the question is "what is the purpose of a best-fit line
and <br>
how do you use it"? <br>
<br>
A best-fit line is used to average out the random error in your data
<br>
points. It's like taking a graphical average of your data. This
line <br>
may not even go through a single data point, unless you force it <br>
through a point. We do this for the Beer's Law graph for BAR <br>
because we know when the concentration of your reactant is zero <br>
the absorbance should be zero so this should be a very good data <br>
point, plus you're taking this point several times when you
calibrate. <br>
<br>
Since the line averages out the random error in your experimental <br>
data, the points on the line are considered better than your actual
<br>
data. When you calculate the slope you should use points from the <br>
line and not the actual data points, unless the line goes through
one <br>
of them. This will give you a better slope than using your data
points.<br>
Excel essentially uses the points on the line for the slope. <br>
<br>
This is what you should have done for the BAR pre-lab. Graphing <br>
and other numerical treatments are in Appendix D (Treatment of <br>
Numerical Data). <br>
<br>
For the report you should use Excel. You should include the (0,0)
point <br>
with your data and plot it with the ones for the solutions you
created in <br>
Part C. This should give you a total of 5 points for your Beer's
Law <br>
graph. Have Excel do a linear best-fit. It allows you to force the
line <br>
through a point (chose the origin, the (0,0) point). Have it print
the <br>
equation and R<sup class="moz-txt-sup"><span
style="display:inline-block;width:0;height:0;overflow:hidden">^</span>2</sup>
value. Your eqn should not have an intercept. If <br>
it does it means you didn't force it through the origin. You will
use <br>
the slope from this eqn as your Beer's Law slope (constant) for<br>
calculating the conc. of Allura Red at each time. <br>
<br>
Dr. Zellmer
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