Ch 206 Lab Help

Determination of the Ionization Constant of a Weak Acid

Here are some tips for using Spartan:

Follow the instructions in the lab manual to construct your assigned weak acid and nitric acid. Have Spartan display the calculated charges on each molecule, and record the charges on the acidic proton and the oxygen to which it's attached. Also have Spartan display the O-H bond length for the O-H group which functions as the acid.

After you have successfully modeled your weak acid and nitric acid, next model the anion (conjugate base).

Build the model as you did before except the oxygen with the negative charge must be treated differently.
To represent this oxygen in nitrate ion, select the expert (Exp) toolset and click on oxygen on the periodic table. In the list of bond types below, choose the bond which looks like a single dash and a dot (-*, not -*-) , and add this oxygen to the nitrogen or carbon.
From the setup menu, select calculations. Change Total Charge from neutral to anion.
Submit your calculation and display the electrostatic potential map as directed in the lab manual.

Here are some tips for the lab report.....

You are to determine the K_a value for either formic or propionic acid by the method of half- neutralization. You did two titrations in order to find the volume of 0.025 M KOH required to neutralize a 5.00 mL sample of your acid. Begin with the usual brief but informative intro and explain the mathematics here a litte bit!

Continue with the usual procedure section - don't forget to mention that you used Spartan to do some things in this lab!

Report your measurements in Table 1 in the results section, like this:

Table 1. Experimental data for neutralization of a weak acid.

Trial Volume acid (mL) Volume 0.025 M KOH added (mL)

1 5.00

You then added enough base to take a sample of your weak acid exactly halfway to the equivalence point. At this point, pH = pK_a (you might mention why this is so), and since pK_a = -log(K_a), you can find the K_a for your weak acid. Report this value in your results section, and compare it (using a proper citation) to the accepted values (at 25^oC) in Appendix D of your text in your discussion section. Since you have a literature value with which to compare, an error analysis is appropriate.

Also in the discussion section, discuss your Spartan results (try to copy and paste some graphics in here to help support your arguments. Label and refer to them as Figures.) Unless you're printing in color, don't mention that one area was 'redder' than another!

Discuss why nitric acid is a strong acid, while your carboxylic acid is weak. Discuss in terms of the following:

polarity of the acidic O-H bond in each species (as estimated by the partial charges in Spartan)
length of the acidic O-H bond as a measure of acid strength
charge distribution in the conjugate bases (i.e, the anions) - is one anion better able to 'spread out' the negative charge than the other? Why? How does this correlate with the base strength of these species?
Which of these three factors appears to be important in explaining the difference in acid strength between your weak acid and nitric acid?

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