questions about exp 12

[Zellmer, Robert]

I always get questions about exp 12.  Here are the questions and answers as if
you collected the data:

1)  For the density of H2O in part B what if my temperature is between
       those listed in the table on page 83, 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)  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).

3)  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.

4) 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 in report question 5.

5) When calculating "R" in Part E use the actual molar mass based on your
     molecular formula from Part D.  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 from part D.

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 81 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
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