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I've received a few questions about exp 12. Here are the questions<br>
and answers:<br>
<br>
1) For the density of H2O in part B what if my temperature is
between<br>
those listed in the table on page 85?<br>
<br>
Here are your two choices:<br>
<br>
a) You can look up the density of water in the CRC manual.
You can<br>
find a link to the on-line CRC on my web site (either from
my homepage<br>
or the "Helpful Tidbits" link on our 122 webpage (not
Carmen). This<br>
is in Section 6: Fluid Properties, Standard Density of
Water. There the<br>
density is listed at every 0.1 C to 7 decimal places.<br>
<br>
b) You can do a linear interpolation between the nearest two
temperatures.<br>
In this case this may not give the best result because for
the particular<br>
temp given of 15.8 C is between 10 and 20, a ten degree
range. It<br>
does work pretty well for the densities at temps between 20
and 30 since<br>
those are listed at every one degree. Even for temps in
this range the<br>
error doing the linear interpolation using the info in the
manual compared<br>
to that given in the CRC is only about 0.015 %. This isn't
going to effect your<br>
final calculated experimental MW in a significant way.<br>
<br>
By the way, what's a "linear interpolation"? That means
you take the<br>
two points (the two density and temp points) and you fit
them to a line<br>
(i.e. get an eqn for a linear line). You can use this eqn
and the temp.<br>
you have to then find the density at your temperature which
is between<br>
the two temperatures you used for the linear interpolation
fit.<br>
<br>
2) For Part D you are supposed to determine the molecular formula
from the<br>
emp. form. by finding the ratio, MW/EFW (molec. wt. divided by
emp. form.<br>
wt.). The manual tells you to round down, even if you get
something like 1.9. <br>
<br>
This can lead to two problem cases. <br>
<br>
a) The ratio is less than 1. Check to make sure you did the
calculations<br>
correctly for the MWs (Part C) and got the correct emp.
form. If so<br>
you most likely made an experimental error (maybe waited
too long<br>
when reweighing the flask after cooling it so some of your
sample<br>
vaporized and escaped from the flask). You can't round
down to zero<br>
so you have to round up to 1 (molecular and emp. formulas
are the same). <br>
<br>
b) You round down as told and you get the wrong molecular
formula. <br>
How would you know the formula might not be correct? When
you <br>
have a compound containing C and H or C, H and O the
MOLECULAR<br>
formula MUST have an EVEN number of H atoms. You can have
an odd<br>
number of H atoms in an empirical formula. However, You
can NOT have <br>
an ODD number of H atoms in the MOLECULAR formula so a
molec.<br>
formula such as C3H7 can not be correct (this is fine for
an emp. form.).<br>
Perhaps you got this because your ratio in Part D was 1.8
and you<br>
rounded down to 1 (as told to do in the manual). In this
case you should<br>
really round up to a ratio of 2. That would give C6H14. I
can't say that's<br>
correct. You may still have done something else wrong.
However, I can<br>
say the molecular formula can't be C3H7 so reporting it as
C6H14 would<br>
be better. <br>
<br>
3) In Part C, what if two of the MWs are really close and one of
the MWs is really<br>
different than the other two? What can you do?<br>
<br>
a) You can leave out the one which is really different.
Technically, you should do<br>
an error analysis as explained in appendix F and at the
following link on my web<br>
page ("Laboratory" link),<br>
<br>
<a
href="http://chemistry.osu.edu/%7Erzellmer/chem1250/lab/App_D_122_lab_manual.pdf"><b>Treatment
of Numerical Data (Error Analysis, sig. fig., graphing)</b></a><br>
<br>
This may apply for other exps as well (exp 16 is another
example with three trials<br>
for determining a MW).<br>
<br>
4) What are reasonable values for "a" and "b" in the VDW eqn? <br>
<br>
Remember what these constants represent. There is a table on
page 83 of the<br>
manual and Table 10.3 in Chapter 10 of the textbook which have
"a" and "b"<br>
values for several substances. <br>
<br>
5) Getting a negative "a" value. Does this make sense? <br>
<br>
Think about what "a" stands for in the VDW eqn. It's about the
attractive forces<br>
between particles which cause the measured pressure to be LOWER
than the<br>
ideal pressure. Look at the eqn and think about whether it
could be negative. <br>
<br>
Remember, you are solving for "a" by simply rearranging the
eqn. ALL the error<br>
in every value we substitute into the eqn winds up in "a".
This is not the best way<br>
to find the "a" value. <br>
<br>
Dr. Zellmer
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