Subject: Exp 14 and using Excel for Exp 14

When recording the volumes and temps you can estimate to
one decimal place.  Your values taken in lab should reflect this.
For example, if your volume on the syringe is exactly on 8 mL
you should record it as 8.0 mL.  If your temp. is exactly on 30
it should be recorded as 30.0.  The barometric pressure should
be reported to 1 decimal place.  If you don't do this both in the
notebook and the report sheet you lose points.  Don't just round
to the nearest mL, degree or torr.

If you go to the "Laboratory" link on the class web page you
will find lots of info there as well.  You will be collecting
5 temp. and volume readings for your compound.  The 5 volumes
are listed in the manual and at this link.  The volumes you use
depend on the particular compound you are assigned.  The volumes
you use don't have to be exactly those listed in the manual, just
close to those.  When you get close to one of the volumes you then
try to adjust the heat so the volume remains constant for about
30 seconds and then record the temperature.

For Exp 14 you will need to do quite a few calculations and a bit of
graphing.  You should use Excel to do both.  I have a link for how to
use Excel for exp 14.  I would highly suggest you use it for exp 14,
even if you don't know how.  You should know by now since you should
have used it for three reports.  It will come in handy not just for this
exp but especially for exp 16 where you have to do a lot of calculations
and graphs.  Even if it takes you awhile to figure out how to use Excel
it will save you time in the long run.  If you have questions you are
more than welcome to e-mail me or bring in your Excel files for me to
look at. If you e-mail or bring your Excel files I need them in the
older format (Excel 2003, .xls).

By the way, I was using Windows XP and Excel 2003.  Things look
different if you are using Excel 2007 but you can still figure
it out by looking at my example.

In my example I show how to set up equations in Excel to do the calculations
for you.  The only equation I don't show you how to set up is #6 that
relates P_s to P_b, V_r, T_r, V and T) in the "Analyzing Your Data" section
in the manual (of course it's the hardest one, but hey you have to do something
for yourself).  I also show how to do the graphing.  One of the hardest things
to figure out how to do is to plot more than one set of data on a single
graph but I show you how to do this.
 
If you make your Excel worksheets look like your report sheets (including
the headings), use them as your data sheets and include them in the report
before uploading it.  There is no need to copy all the data to the report
sheets.

Your graphs should take up essentially the whole page with one graph to a
page (whether using Excel or another graphing program).  You are not allowed
to make hand plots using graph paper.  They must be electronically produced.
The axes should be set up so your data points occupy most of the space on the
graph (no large empty areas - spread the data points out).  It's easiest to
just make the margins zero inches (don't worry if the Excel complains about
the printer - you're not printing them).  Landscape mode is the default in
Excel and is usually the best orientation to use.  You need to have a title
and axes labels (with proper units).  Don't make the font for these things
so large it takes up a lot of space, a 12 point font will be big enough for
the labels and numbers on the axes (14 point for titles is fine).

You need to make sure when you pull the graph from Excel into your Word
document the correct margins you set in Excel are still set in Word.
Word tends to change the margins from Excel to be what you have it set
in Word.  This will shrink the graphs so they no longer occupy most of
the pain.  There is a way to include the graphs from Excel in Word so
the margins for the graphs stay what you set them to in Excel.  You could
also copy them from Excel and paste in the Word doc but that won't fix
the problem with them now having the margins in word or being on a
portrait page rather than in landscape.  I've explained elsewhere how to
do this.

When you have more than one set of data points on a graph you need a legend
indicating which data points belong to which compound.  This will be the
case for this graph where you will plot the data for the three compounds.
You do not need a legend for the data when there's only 1 set of data plotted.
Always do a scatter graph, not a line graph.  Then you will do a "trend line"
for the data points for each compound.  In this case fit the points to a
linear fit.  Print the equation from Excel and the R^2 value next to each
line.  You will be using the slope given by Excel.  The R^2 value is one
indicator of how good the fit is.  The closer R^2 is to 1 the better the
fit is.  If your graphs are not done properly you WILL have points deducted
for each mistake for each graph.

Even though Excel is doing the calculations for you (if you set it up to
do so) you still have to show the sample calculations.  You only have to
show the sample calculations for your compound.  For the sample
calculation of Hv (heat of vap) you should show a sample calc. for slope.
Take two points from the line for your compound use those to do a "rough"
sample calc. for the slope.  Don't use your actual data points for this
calculation.  Don't worry about it's accuracy since you will ultimately
use the slope from the equation Excel gives.  By the way, this Excel
slope will likely have too many sig. fig.  Your slope really should have
only 3 s.f.

You will be sharing data with two other students.  This is as far as
any collaboration goes.  You are not to work together in the lab or
afterwards on the reports.  If you use Excel, you are not to share the
Excel files with each other.  You must do the work in Excel yourself.
Don't think we can't tell when students share Excel files.  We can and
that's academic misconduct.

Remember, my examples for exp 14 and exp 17 are just examples.  I have
not done everything you need to do.  I've left things out of the table
(such as how to program in eqn 6 for exp 14) and some things off the
examples of the graphs (units, etc.).  You need to figure out some of
these things for yourself.

You will need to look up the normal boiling points for the three compounds
from the "CRC Handbook of Chemistry and Physics".  I have a link to
instructions on how to access the online version of this handbook on my
homepage.  The vapor pressure is 1 atm (760.0 mm Hg) for this point.

For each compound you will use eqn. 6 to determine Ps from your T and V
data.  Make sure your T's are in KELVIN.  For each compound you have 7 lines,
5 lines with T and V data and two lines for the boiling point data (one
for your experimentally determined b.p. and one for the normal b.p. from
the literature).

When calculating Ps using eqn. 6 your Vr always has only 2 s.f. so the
Vr*T/V*Tr has only 2 s.f. so Ps winds up with only 2 s.f.  You must report
it on your report sheets this way.  When taking the log of a number the
sig. fig. info winds up to the right of the decimal place.  For example,
if your number is 4.8 x 10^2 (2 s.f.) then log(4.8 x 10^2) is 2.6812.
This should be reported as 2.68.  The "2" to the left of the decimal comes
from the power of 10 (which has no s.f. info).  The "68" to the right of
the decimal signifies the number of s.f. in the original number.  In other
words, when taking the log of a number your s.f. info winds up to the right
of the decimal and is given by the number of decimal places shown.
Appendix A in the textbook also covers this.  Since your Ps values have
2 s.f. (assuming you recorded Vr and the other volumes to 1 decimal place)
your ln(Ps) whould have 2 decimal places.

You do NOT use eqn 6 for Ps of the boiling points.  You will have two b.p.

You get the normal b.p. (literature value) from the CRC ("CRC Handbook of
Chemistry and Physics").  I have a link to instructions on how to access
the online version of this handbook on my homepage.  The vapor pressure (Ps)
is 1 atm (760.0 mm Hg) for this boiling point.

You are also measuring the boiling point at the prevailing barometric
pressure on the day you do the exp.  This will likely not be the same for
different days or maybe even morning and afternoon labs on the same day.
It is not likely to be the same as the normal b.p. (unless of course the
pressure in the room is 760.0 mm Hg).  A liquid boils when it's vapor
pressure equals the barometric (atmospheric) pressure.

For the b.p. data points you are using 760.0 (literature value) and your
barometric pressure (which you should have recorded to 4 s.f.).  Thus your
ln(P) should have 4 decimal places.  

When using eqn 6 for determining Ps you MUST use Kelvin.  If you
look at the preceding lines in the derivation of eqn 6 you will see
it comes from the Combined Gas Law and you MUST use Kelvin in this
eqn.  If you don't use Kelvin you are likely to get negative numbers
for Ps and you can't have negative pressures, let alone take the log
of a negative number.

For your T and 1/T using Celsius you should have recorded the T's to
1 decimal place.  If you did, you have 3 s.f. for your temperatures.
When you add 273.15 to convert to Kelvin you know your T in kelvin to
1 decimal place.  I will leave it to you to figure out how many s.f.
that would be for T and 1/T using kevlin.  If you only recorded your
temps to the units place and didn't write them to 1 decimal place your
s.f. in kelvin shouldn't be reported to more than the units place.
This means you have fewer s.f. for T and 1/T.

You MUST INCLUDE the BOILING POINTS on your graphs (there should be 7 data
points for each compound) and clearly LABEL the boiling points on the
graphs for each compound.  Also, for each line (set of data) the points
for the b.p. should be the highest and furthest to the left  (i.e. it should
be the one with the largest ln(P) and smallest 1/T).  If this isn't the case
(one of your measured Ps values has a higher ln(P) than that of the b.p.) that
point is likely incorrect.  For your compounds, the b.p. you get from the CRC
will undoubtedly be better than any of your data points for Ps and your
experimental b.p.  If your b.p. data points are not the highest and furthest
to the left you should discuss why this shouldn't be the case in your
discussion section.  You should also compare the two b.p. values.
If your experimental b.p. is well out of place (way to the right of where
it should be) you should have it on the graph but do not include it in the
best-fit line.

When writing the report, include your results for the heat of vap. for
the 3 knowns and the two boiling points for all compounds.  State which
b.p. is the normal b.p. and the barometric pressure at which you measured
your experimental b.p. (eg. 98.1 C at 747.1 torr).  For the the heats of vap.
state whether you got the correct order for the compounds based
on their attractive forces (explaining your reasoning).  If you didn't
get the expected trend, state this and then state what it should have
been (again explaining your reasoning based on the AF for the compounds).
You need to discuss what the AF are (be specific) and how these affect
the heat of vap. and the b.p.  You won't know what the heats of vap. for
any of the compounds should be but you should state whether they seem
reasonable (you can find them in the CRC or on-line and use this info).
You can base this on what you were told in class about the magnitudes of
delH_vap.  You can also get an idea about this from the textbook ("A Closer
Look" on page 455 of the 14th edition, page 463 of the 13th edition,
page 444 of the 12th edition, page 456 of the 11th edition and page 462
of the 10th edition dealing with the Clausius-Clapeyron Eqn and some
homework problems).  Don't forget to state reasonable sources of error,
both inherent errors and mistakes you may have made leading to errors.


For sample calculations you should do the following for one line for one
compound:

1/T

Ps (using eqn 6 in the manual)

A sample calculation for the slope of the line for your compound from the
graph.  This does not mean to simply write the eqn. from Excel (if using
Excel) or the slope from the eqn.  Take two points from the line and show
a calc.  Don't worry about its accuracy.  You will ultimately use the slope
from the line given by Excel for the calculation of H_vap for all compounds.

Delta(Hv) for your compound using the slope calculated by Excel.

Telling us Excel did the calculations will NOT count as sample calculations.
Printing out the eqns from Excel will NOT count as sample calculations.


For further information concerning exp 14 see the "Laboratory" link on
the class web page.


Dr. Zellmer