questions about exp 12

[robert zellmer]

I've received a few questions about exp 12.  Here are the questions
and answers:

1)  For the density of H2O in part B what if my temperature is between
        those listed in the table on page 77, such as 15.8 C?

Here are your two choices:

      a)  You can look up the density of water in the CRC manual. You can
          find a link to the on-line CRC on my web site (either from my 
homepage
          or the "Helpful Tidbits" link on our 1250 webpage (not 
Carmen).  This
          is in Section 6: Fluid Properties, Standard Density of Water.  
There the
          density is listed at every 0.1 C to 7 decimal places.

      b) You can do a linear interpolation between the nearest two 
temperatures.
          In this case this may not give the best result because for the 
particular
          temp given of 15.8 C is between 10 and 20, a ten degree range.  It
          does work pretty well for the densities at temps between 20 
and 30 since
          those are listed at every one degree.  Even for temps in this 
range, the
          error doing the linear interpolation using the info in the 
manual compared
          to that given in the CRC is only about 0.015 %.  This isn't 
going to effect your
          final calculated experimental MW in a significant way.

          By the way, what's a "linear interpolation"?  That means you 
take the
          two points (the two density and temp points) and you fit them 
to a line
          (i.e. get an eqn for a linear line).  You can use this eqn and 
the temp.
          you have to then find the density at your temperature which is 
between
          the two temperatures you used for the linear interpolation fit.

2) For Part D you are supposed to determine the molecular formula from the
      emp. form. by finding the ratio, MW/EFW (molec. wt. divided by 
emp. form.
      wt.).  The manual tells you to round down, even if you get 
something like 1.9.

      This can lead to two problem cases.

      a) The ratio is less than 1.  Check to make sure you did the 
calculations
          correctly for the MWs (Part C) and got the correct emp. 
formula.  If so,
          you most likely made an experimental error (maybe waited too long
          when reweighing the flask after cooling it so some of your sample
          vaporized and escaped from the flask).  You can't round down 
to zero
          so you have to round up to 1 (molecular and emp. formulas are 
the same).

      b)  You round down as told and you get the wrong molecular formula.
          How would you know the formula might not be correct?  When you
          have a compound containing *C and H or C, H and O* the *MOLECULAR*
          formula *MUST *have an *EVEN *number of H atoms.  You can have 
an *odd*
*number of H atoms *in an *empirical formula*. However, You can *NOT *have
          an *ODD **number of H atoms *in the *MOLECULAR *formula so a 
molecular
          formula such as C3H7 can not be correct (this is fine for an 
emp. form.).
          Perhaps you got this because your ratio in Part D was 1.8 and you
          rounded down to 1 (as told to do in the manual).  In this case 
you should
          really round up to a ratio of 2.  That would give C6H14.  I 
can't say that's
          correct.  You may still have done something else wrong. 
However, I can
          say the molecular formula can't be C3H7 so reporting it as 
C6H14 would
          make sense.  Make sure you discuss what may have caused this 
error and
          why you rounded up rather than down.

3)  In Part C, what if two of the MWs are really close and one of the 
MWs is really
      different than the other two?  What can you do?

      a)  You can leave out the one which is really different. 
Technically, you should do
          an error analysis as explained in appendix F and at the 
following link on my web
          page ("Laboratory" link),

*Treatment of Numerical Data (Error Analysis, sig. fig., graphing)* 
<http://chemistry.osu.edu/%7Erzellmer/chem1250/lab/App_F_1250_lab_manual.pdf>

      This may apply for other exps as well (exp 16 is another example 
with three trials
      for determining a MW).

4)  In Part C, can you gain a s.f. in the average when from only 3 data 
points?  No!
      This was discussed in class.  While you can gain s.f. when adding 
numbers, and then
      ostensibly in the average, when dealing with real data this isn't 
safe to do.  Think of
      doing a best-fit line.  The purpose of this is to average out the 
random error in your
      data.  What if you have only 2 data points?  The line would go 
through both points
      and not average out any error in the data.  Adding a third point 
wouldn't help a lot.
      So how many data points do you need to safely report an extra 
digit?  That's tough
      to say as it depends on the data itself.  I stated in our class if 
you have 4 or more
      data points and you're taking an average you can report an extra 
s.f..  The same for
      data from a graph.  If fitting 4 or more points you can report an 
extra s.f.

5) When calculating "R" use the actual molar mass based on your 
molecular formula.
      Use the V from Part B and the P, T and n from each trial in Part A.

6) When calculating "b" in the VDW eqn in Part F use the actual (exact) 
molar mass
      from the molecular formula you got.

7) When calculating "a" in the VDW eqn in Part F use the date from
      your best trial in Part A based on the % error for your R values.  Use
      the actual value of R (not the calculated value), the actual (exact)
      molar mass from the actual molecular formula you got to get the moles
      and the P and T from the best trial.

8) What are reasonable values for "a" and "b" in the VDW eqn?

      Remember what these constants represent.  There is a table on page 
75 of the
      manual and Table 10.3 in Chapter 10 of the textbook which have "a" 
and "b"
      values for several substances.

9)  Getting a negative "a" value.  Does this make sense?

      Think about what "a" stands for in the VDW eqn.  It's about the 
attractive forces
      between particles which cause the measured pressure to be LOWER 
than the
      ideal pressure.  Look at the eqn and think about whether it could 
be negative.

      Remember, you are solving for "a" by simply rearranging the eqn.  
ALL the error
      in every value we substitute into the eqn winds up in "a". This is 
not the best way
      to find the "a" value.

Dr. Zellmer
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osu.edu/pipermail/cbc-chem1250/attachments/20191019/f02dcdba/attachment.html>