Exp 14 help, Ps values from eqn 6, Ps for b.p.

[robert zellmer]

I've received questions from several of you about getting negative
values for Ps from eqn 6 in the manual and what to use for Ps
for the boiling points.  I think this info is in my help file for exp 14
on my web page and it is in the manual (paragraph below eqn 6).

1) Exp 14 help: For help with exp 14 see the following links,

*Laboratory* 
<http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/lab_1220.htm><http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/lab_1220.htm>

*Exp 14 - Help for Exp 14* 
<http://chemistry.osu.edu/%7Erzellmer/chem1220/faq/exp14.txt>


I've also received a few e-mails about not understanding what to do for
the on-line data entry program.  You need to thoroughly read the discussion
and data analysis section and write the procedure first.  You will be given
data as if you collected it for your unknown.  You will then use eqn. (6) to
calculate the pressure of the sample, Ps, using the temperature, volume and
barometric pressure data given.  For the boiling pt you don't use eqn 
6.  The
Ps for the b.p. is simply the barometric pressure at which you 
determined the
b.p. in lab.

1) Negative Ps values?  Wrong unit for T.  Remember this

KELVIN, KELVIN, KELVIN

You pretty much always need to use kelvin temp. scale in eqns.
The only time it's really safe to use Celsius is when it's a
delta(T) (change in temp).  This is because the size of the unit
in Celsius and kelvin is the same.

2) You don't need eqn. 6 for the V.P. at the boiling point. A
     substance boils when it's V.P. equals the prevailing atmospheric
     (barometric) pressure for the day.  Thus, the b.p. for a liquid is
     different throughout the day as the barometric pressure changes.
     The normal b.p. is the boiling temperature when the atmospheric
      pressure is 1 atm (760.0 from the manual, use 4 s.f.).

     You get the normal b.p. from the CRC.

3)  Use the data for YOUR compound in the post-lab data-entry.
      You won't enter the partner's data on-line but it will be on the
      report sheet.

Remember, I have a help file for this exp and an example for
using Excel and the graph for the 3 known compounds.  There
should be two graphs, one with the data for the 3 knowns and
one with the data for your unknown.

Dr. Zellmer
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