exp 17 questions, sample calculations and sig. fig. for slopes, graphs. etc.

[robert zellmer]

I've received a few questions about exp 17.  I have may have already
addressed them in a previous e-mail or my help links but here they are
one more time.

1)  In my on-line example I used different conc. than what you used in the
     actual exp.  Do NOT use the conc. I have in my example.  Use yours.

2)  Your initial concs. for each solution are different.  You need to do a
      dilution calc (M2*V2=M1*V1) to get the initial Cr^3+ conc. for each
      solution at the moment of mixing (with the EDTA soln.).  The 
initial Cr^3+
      conc. and the conc. of the CrEDTA- product in the boiled solns are the
      same since all the Cr^3+ is converted to the product in the boiled 
solns.

3)  If you are using Excel and have your data tables set up so they look 
like
     those on the report sheets (headings, units, etc.) you do not have 
to recopy
     all the data to the report sheets.  You can turn in your Excel 
sheets.  However,
     you still have to turn in the blank report sheets with the lab 
report.  Make sure
     you have the correct number of s.f. and units in your tables (and 
graphs).

4) For the sample calculations,  see item # 12 in the following link,

*Exp 17 - Help for Exp 17* 
<http://chemistry.osu.edu/%7Erzellmer/chem1220/faq/exp17_help.txt>

5) You need to report the correct number of s.f. for the rate constants
      To set the number of s.f. for the slopes in your graph you
      need to right click on the equations on the graph.  Then choose 
"format
      equation", then choose "number" and then set it to enough decimal 
places so
      the slope has the correct number of s.f.  For most of you 
measuring the time
      to the minutes using the wall clock, the slope should have 3 s.f.

6) For Part C, item #6 is asking for the Average value of your rate 
constant
     AND the avg. deviation.  See the link in the "Laboratory" link 
which discusses
     the treatment of numerical data,

*Treatment of Numerical Data (Error Analysis, sig. fig., graphing)* 
<http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/App_D_122_lab_manual.pdf>

7)  I've also received questions about the graphs.

     If the slope of your Beer's Law plot is a lot lower than 200 or a
     greater than 250 you may have done something wrong.  This
     graph should have 5 data points (4 solutions and the origin (0,0))
     and be forced through the origin (an option when do your trend line).
     You will know if you didn't do this if your eqn. has an intercept.

     If using Excel, you need to put the data for ALL 4 solns on the zero,
     first and second-order graphs.

     Take a look at my help files and the Exp 17 Excel example.  Make
     sure your graphs take up the whole page (one per page) and your
     data points occupy pretty much the whole graph area.  Take a good
     look at the picture I have for graph 2 (zero-order). It will show 
you an
     example of what a good graph should look like,

Graph 2 
<http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/exp17/exp17_web_graph2_ex.pdf> 
- Examples of a good graph

     These examples are for the zero-order plot.  This is actual
     data and graphs from a previous year that I cleaned up.
     You can use an exponential or parabola for the zero-order plot.
     (which ever seems to give a better fit, usually exponential).
     Do *NOT *use a *linear *fit for the zero-order plot.

     Note two lines cross toward the end.  They shouldn't cross. There
     was something wrong with the data toward the end of the run
     for one of the solutions.  When this happens you will notice in
     graphs 3 or 4 (which ever produces the most parallel lines) the
     slope for one of the lines which cross isn't as similar to the slopes
     for the other lines (not as closely parallel).  In this case you should
     report all four rate constants on the report sheet but might consider
     not including the "bad" rate constant in your average.  It depends
     on how different it is from the other three.  You should discuss this
     in the Discussion section of the report.

     Also, if your zero-order graph shows all 4 lines pretty much
     emanating from the same point (initial conc) or crossing early
     on (at early times) you've likely made a mistake with your
     calculations for the initial concentrations.  Your initial 
concentrations
     for each solution should be different.  Also, your data points for each
     should all be starting at pretty much the same time and be pretty
     much lined up vertically as time proceeds (see graph 2).

     For both graphs 3 and 4 fit the data to LINEAR trend lines.  Do
     NOT go back and fit either to something other than linear.  You are
     looking to see which graph has lines that are more closely parallel.
     Generally speaking if you have good data you will see a distinct
     difference between the two graphs.  Also, which ever graph has
     lines which are more closely parallel will often produce a better fit
     to the points.

     See the following link for examples of what you might see,

Graphs 3 & 4 
<http://chemistry.osu.edu/%7Erzellmer/chem1220/lab/exp17/exp17_web_graphs_3_4_exs.pdf> 
- Exs of what Graphs 3 & 4 might look like

     For graph 3 (1st-order) the label for the axes will likely be at the
     top of the graph.  You can move it to the bottom.  You actually
     have to right-click on the Y-axis, choose "Format Axis" and then
     choose "Value (X) axis crosses at:" or "Horizontal axis crosses:"
     and set this to the minimum value on the Y-axis.  It should move
     to the bottom of the graph.

     Set all the graphs to be printed in *LANDSCAPE *mode (normally the
     default for Excel).  This gives better looking graphs.

     Don't forget to discuss how you chose the order based on
     graphs 3 and 4.   Also, discuss why one of the graphs should
     have lines which are parallel. Don't forget your rate constants
     or the rate law.

     Please remember, my Excel example is just that, an example of what
     to do and how to do it.  It is not a complete example.   I did NOT use
     the same initial concentrations for the Cr^3+ as you are using. It was
     also produced a long time ago when most students were still graphing
     this by hand.  The example was done based on the manual at the time
     when we still used Excel to do it the same as someone doing it by
     hand.  I've changed some of my instructions in my Excel example to
     explain what to do if doing it by hand or using Excel.

     I hope this helps.


Dr. Zellmer
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