Subject:  Exp 1 help

I always get questions about exp 1.  Hopefully this will answer them.

1)  Temperature on the graphs can be in Celsius.

2)  In the lab, you will be working with oxalic acid as a solute into
    parts A and B.

3)  Part of part A is done in the lab and part of the data will be given
    to you.  You collect data for solubility of oxalic acid in H2O and
    H2O/dioxane solvent.  You're given data for potassium dichromate.
    It will be at the bottom of the e-mail you get after completing the
    pre-lab.

    This data will be graphed.

    You can do this with Excel.  Don't use a linear fit.  Use one of the
    fits that will give a best-fit curve (polynomial, exponential).  Use
    the trendline that gives the best fit and is a "smooth" curve.  It
    shouldn't undulate up and down but look like Fig 1.1 on the first page
    of the discussion section for exp 1 in the lab manual.

3)  Part B data (H2O/dioxane solvent) should be on the same graph as the
    Part A (H2O) data.  However, it is a separate trend line.  Again, do
    a best-fit curve (whether by hand or computer generated).  This curve
    should have 4 data points.

4)  You should wind up with the two curves on graph 1 (solubility curve),
    one for the H2O data and one for part B data (H2O/dioxane solvent).
    One line should be above the other.  Which one depends on your solute.

5)  Graph 2 is Temp. vs. Mole Fraction of Solute for the H2O solvent only.
    This should have 7 data points.  A logarthimic, ln, trend line works
    well if you have decent data.

6)  When you recorded the temperatures in lab you should have been able to
    estimate them to 1 decimal place.  Thus the temps should have 3 s.f.  If
    you did not do this don't simply add a "0" after the decimal point.

7)  Your volumes should have been measured with the 10.00 and 5.00 mL pipets.
    This means your volumes should have been reported to 2 decimal places
    and should have 4 s.f.  If you used a graduated cylinder to measure any
    of the volumes you can not report your volumes to 2 decimal places.

8)  Your masses should have been done using the anayltical balance.  This
    means they should have been reported to 4 decimal places and thus have
    5 s.f. (since the masses were between 1 and 7 grams for all parts).

9)  When weighing the two types of solids use the lower end of the range
    given in the manual, especially for the potassium dichromate.  See my
    "Laboratory" link (exp 1, SOL, info) for more specifics.  If you don't
    you could have problems getting the solute to dissolve and will be in
    lab longer.

10) You do not need the density of the H2O or H2O/dioxane at different
    temperatures.  You are measuring the volume of each solution at room
    temp and can use the densities at room temp.  Once you have the mass
    at room temp it doesn't change at the different temps.  The manual tells
    you what density to use for H2O and the H2O/dioxane mixture.

    The manual tells you to assume 23C for the temp of the water use a density
    of 0.9975 g/mL. If you took the temp of the water and you got something
    other than 23C you could look up the actual density of the water in
    Appendix F of the manual. However, the density for the water/dioxane
    solvent (1.023 g/mL) is likely for 23C so you can just use whats given
    in the manual for both solvents.

11) Since your masses, volumes and density all have at least 4 s.f. your
    solubilities and mole fractions should have 4 s.f.  This is shown in
    the example calculation on the first page of the data anaylisis section.

12) In the Discussion section address how solubility changes with temp.
    Does the trend you see indicate whether the reaction is endothermic
    or exothermic?
    Does this mean heat is a reactant or product in the process?  See
    the equations shown on page 3 and write similar equations in the
    discussion section.  Your equations should show the appropriate
    solute and ions in solution as well as the heat (as reactant or
    product).

13) In your Discussion section you should discuss the solubility as a
    function of temp. for both solvents (using your results and graphs).
    Also, you should discuss the difference in solubility of your solute
    in the two solvents based on your graphs and what you know about the
    various attractive forces between the solute particles, solvent particles
    and solute-solvent particles.  Consider whether things are nonpolar,
    polar (how much polar or nonpolar) or ionic.  Discuss what you find in
    graph 2 (temp. vs. mole fraction).  Finally don't forget a couple of
    sources of error.

    You might even consider seeing what results someone else got for a
    different solute from what you used.  You could discuss this as well
    and it could help with your understanding for the midterm.

    Make sure you address all the Points to Consider.

14) Summarize these factors affecting solubility in the Conclusion section
    and answer the Purpose (Objectives) in a general way (i.e. what did your
    results indicate about how temperature and solute-solvent interatactions
    affect solubility.

15) Look at the following link for an example of what your tables (sig. fig.)
    and graphs should look like.

	http://www.chemistry.ohio-state.edu/~rzellmer/chem1220/lab/exp1_sol.pdf

    You can also find this link on the course home page, either under the
    "Laboratory" link or by clicking on the "Laboratory" link and going to
    the section about exp 1 (SOL).
   
    I used Excel for this example.  I made the tables look like those in the
    manual.  You can actually program in equations to do the calculations.
    You don't need to do that.  This example is mainly to give you an idea
    of what the graphs should look like and what type of fits to use for the
    trend lines.  Remember, do a SCATTER graph (points only - no line).  Then
    do a trend line (stated above and shown in this example).


I hope this helps.

Dr. Zellmer