[cbc-chem1210] More about Exp 1 report, graphs, sample calcs, etc.
robert zellmer
zellmer.1 at osu.edu
Sat Jan 17 09:03:10 EST 2015
I always get a lot of questions about exp 1 and the report. I apologize
to anyone
who's already completed or almost completed the report for exp 1. Here's
what
you should be doing for the report sheet, sample calc., discussion, etc.
1) For the data on the report sheets:
a) line 1: Water temp. (page 8)
If you took the temp for each piece of glassware and it
differed significantly you can use each temp or take an
average temp if they were all about the same. There
were some labs in which the temp changed by a few degrees
degrees from the start of lab to the end of lab. If
you used
a single temp that would introduce error in the accepted
density of the water. If you took only one temp just use
that for your calc. You might think about how this could
affect your results. You can see how much of a 10 degree C
difference makes in the density by calculating it using
the eqn.
for 20 C and 30 C.
b) line 2: Accepted (actual) density (page 8)
Use the eqn on page 8 preceding the data table to calc. the
accepted density at your temp. This is an eqn for a
straight line
relating density to temp. This eqn. is for the density
or H2O between
20 and 30 C. What if your temp was slightly below 20 C
or above
30 C? What can you do? You can go to the CRC Handbook
of Chem
and Physics and look up the table for the density of
water at diff. temps.
Can you still use the eqn on page 8. Well, I did a few
quick calc. based
on that eqn and the values in the CRC between 16 and 40
C. The errors
outside of the range of 20-30 C (given in the manual)
were about the same
as the errors within the 20-30 C range. So the gist of
it is, just use the eqn
on page 8 even if your temp is a little below 20 or
above 30 C.
Make sure you have the correct # of sig. fig. This can
be tricky with this eqn.
After lecture on Mon. or Tue. (depending when you have
lecture) you should
understand how to handle sig. fig. if you don't already.
Show the calc. for this in the sample calc.
c) Data table:
Do the calculations and make sure your sig. fig. are correct.
You only have to show a sample calc. for the measured
density, error and
% error for one line for one piece of glassware. You don't
need to do it for
every single line for every piece of glassware.
You fill out this sheet and turn it in with the report.
This is your
report page or sheet. This is what I labeled "Data" in my
report
example on the class web page. If you use Excel and make
the data
table up in Excel you can print it as the report sheet. You
don't need to
copy all the data over to the data sheet in the manual.
However, you should
still turn in the report sheet from the manual even though
it's blank. If for
some reason you still wish to fill in the report sheet and
turn it in that's fine but
turn in your Excel sheets and let the TA know which one
they should grade
for the report.
You always must make sure you have the correct sig. fig.,
even when using
Excel. It won't let you set sig. fig. but you can set
decimal places to get the
correct sig. figs. I explain how to do this at the
following link:
*Using Excel for Calculations and Graphs*
<http://chemistry.osu.edu/%7Erzellmer/excel/excel.txt>
You can find this link at the following link:
*Using MS Excel for Reports*
<http://chemistry.osu.edu/%7Erzellmer/excel/excel.htm>
2) Graphs
You should have 4 graphs, one for each piece of glassware. Make
sure your graphs look correct, even if using Excel. They
should take up
pretty much the entire page (0.1" margins) and the points
should take up
most of the space on the graph (very little empty space). Your axes
should reflect the accuracy of the measuring device. For
example, for the
buret since you should record the volume to 2 decimal places
your x-axis
(volume) should show 2 decimal places for the labels.
You do NOT need a legend on any of these graphs since there's
only one
set of data per graph. Your title should include the piece of
glassware being
used, but just have "Buret" (as shown in the example in the
manual) is not
an adequate title.
If using Excel have it print the eqn and R^2. If doing it by
hand you should
figure out the slope and intercept and write those on the
graph. Both of
those and the R^2 tell you something. Make sure your slope and
intercept
printed on the graph have enough digits to show differences
between the
glassware and you report them to the correct # sig. fig. in
your report
(results/discussion and conclusion section).
a) Sample calc.
If using Excel show a sample calc for the slope. Use
pts from the line.
Don't use your data pts for this. The pts on the
best-fit line are better
than your data pts. Don't worry if you don't have
grid lines. We just
want a sample calculation. The slope generated by your
sample calc.
will not be used further. In your results and
discussion section use the
slope and intercept generated by Excel.
If hand-graphing show a sample calc for the slope and
intercept. You will
use these in your results and discussion section.
You only have to do this for one of the graphs (not all
four).
3) Answer the questions on page 10. Questions 1-3 should really be
addressed in the
Discussion section. Questions 4-5 can be answered in the
Discussion section or
on a separate sheet of paper. There's no need for you to recopy
the questions.
4) Results and Discussion (and graphs)
If you look at my report example I tell you what should go
here. Use the
questions on page 10 to help guide you.
For this exp. you should discuss the results from the table.
Which piece of
glassware gave the smallest % error, which gave the largest?
Were all the
% errors for a single piece of glassware really close to each
other or not so much?
What does this imply about precision and accuracy? Think about
it based on sig. fig.
Does the # of s.f. seem to effect your answers and what you
think? Do they make sense?
Report the slope, intercept and R^2 from all 4 graphs (pieces
of glassware).
Think of the slope as being your average density for each piece
of glassware.
You should discuss the graphs. What does the slope indicate (m
= D*v + b)?
What does the R^2 indicate about your data? Remember, this is
the "correlation
coeff" and the closer it is to "1" the better the fit (the
closer your points are to the
line). How about the intercept? What should it be? What is
your intercept? What
does your intercept mean if it's not the accepted intercept?
Think about what
would happen to your eqn if the balance had been
miss-calibrated and was
consistently reading a little too high? Would that affect the
slope or intercept or both?
Would it affect the R^2? What if you weren't reading the volume
correctly, i.e. you
didn't look straight on but at an angle but you made the same
mistake each time
so your volume was incorrect by a certain specific amount due
to this? Again,
would that effect the slope, intercept or R^2 or all of them.
Discuss uncertainty (precision) and accuracy. Again, I've said
uncertainty
rather than precision. Precision is how close repeated
measurements are
to each other. Most everyone used volumes around 5, 10, 15 and
20 mL (20,
25, 30, 35 for the beaker). You weren't repeating anything.
(You can look at
how close the densities are.) However, there is still some
relationship to precision.
The mass from a top loading balance can be reported out to 2
decimal places
(e.g. 10.26 g). It has an inherent uncertainty of + or - 0.01
g. If you are
really careful when taking repeated mass measurements your
precision for
your masses should be + or - 0.01 g (within the uncertainty of
the instrument).
You couldn't get more precise than that. What on the graphs
indicate your
precision and accuracy?
Discuss sources of error and how they might affect your
results. How about
the temp. of the water? Was it constant throughout the whole
lab period?
Did you measure it at different times to see if it stayed
constant? How much
might this affect your results? Does the sig. fig. (accuracy)
of the balance
cause any error? How about each piece of glassware? Did you
make a
mistake you know about which could have caused error? If you
have a bad
data point on the graph you don't have to include it in your
best-fit line (a little
harder to do in Excel than by hand). Discuss the fact you
didn't include it.
How did you perform the experiment for each piece of
glassware? For instance,
for the buret, if you added ~ 5 mL each time to the weighing
beaker instead of
emptying and drying the weighing beaker before each volume
added (put in 5 mL,
weighed the beaker, emptied and dried it and then added 10 mL,
etc.) makes a
difference in terms of error and accuracy. How would the
different ways of doing
this make a difference? Were your graduated cylinders TD (to
deliver) or TC
(to contain)? What's the difference and how does that affect
your results? Did you
use the same grad. cyl. for all four measurements or switch
between the two types?
You should have seen the TD or TC at the top of the grad. cyl.
Make sure you have at least 2 inherent sources of error. This
is error which
you have little control over due to the equipment being used or
the way we
had you do the exp. This is not "I spilled some water outside
the beaker".
That's an "oops, I better do this over" moment. What errors
might be introduced
if you added the water a little at a time to get the totals?
For instance, to get
10 mL from the buret you could add 5 mL twice or start over and
add 10 mL
once. Which would be better? Which way did you do it? This
may be something
associated with the instrument (as in the # of s.f. you can get
from the instrument)
but only if it affects the accuracy (s.f.) in your final
results. For instance, if you used
the analytical balance for all the masses (as was best to do)
does it affect your s.f.
for any or all of the measured densities? How about the s.f.
for the volumes?
4) Conclusion
This is 5-6 sentences (depends on the exp) in which you come up
with
what you can conclude from this exp. Essentially answer the
purpose.
Think of this as a summary of the exp. and the results in
sentence form.
5) Summary of sample calc. (as shown on grading rubric).
a) calc for the accepted density of water (2nd line on report
sheets, page 8).
b) calc. for measured density for one piece of glassware for one
mass-volume pair
c) calc. for error for one piece of glassware for one mass-volume pair
d) calc. for % error for one piece of glassware for one mass-volume
pair
e) sample calc for graphs (just using one of the four graphs):
1) slope whether by hand or using Excel (use points from one of
your lines)
You only have to show a single sample for this for the graphs.
You
need to report slope and intercept for each of the 4 graphs.
This is
one reason to use Excel. Excel will give better results than
you can likely
get by doing it by hand.
Please indicate which piece of glassware and lines for the table
you're using for the
sample calc. and indicate which graph you are using for the slope calc.
I hope I haven't left anything out and I hope this helps.
Dr. Zellmer
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