<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
</head>
<body text="#000000" bgcolor="#FFFFFF">
I've had a few people ask about the graph and calculating the slope<br>
for the pre-lab for BAR, after the fact. There seems to be a<br>
misunderstanding of the purpose of a best-fit line and how to get<br>
the slope. People often take data points for the slope instead of<br>
points from the line. Lets get this cleared up for the report.<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. You will force it through the origin for the
Beer's<br>
Law graph for BAR because we know when the concentration of<br>
the reactant is zero the absorbance should be zero so this should<br>
be a very good data point, plus you're taking this point several
times<br>
when you calibrate before taking readings for each solution. <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 and not
your<br>
actual data points (unless the line goes through one of them). <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. For the Beer's Law graph, you<br>
should include the (0,0) point with your data and plot it with the<br>
ones for the solutions you created in Part C. This should give you
a<br>
total of 5 points for your Beer's Law graph. Have Excel do a linear<br>
best-fit. It allows you to force the line through a point (chose
the<br>
origin, the (0,0) point). Have it print the equation and R<sup
class="moz-txt-sup"><span
style="display:inline-block;width:0;height:0;overflow:hidden">^</span>2</sup>
value.<br>
Your eqn should not have a nonzero intercept (it should look like<br>
y = m*x). If it does (looks like y = m*x + #) it means you didn't
force<br>
it through the origin. You will use the slope from this eqn as your<br>
Beer's Law slope (constant) for calculating the conc. of Allura Red<br>
from the absorbance at each time. <br>
<br>
For the first and second-order plots do NOT force the lines through<br>
the origin or any other point.<br>
<br>
Dr. Zellmer
</body>
</html>