pH Titration

You are given a solution of a diprotic acid whose pKa values are 4.0 and 7.0.
You have a solution of NaOH to use in a titration experiment.

Plot the pH versus added NaOH (in equivalents) using Excel.

You should increase the amount of NaOH by 0.2 equivalents at a time.

Note:In contrast to a "wet lab" titration where the pH would increase step-wise with each addition of NaOH, this titration uses a new solution of acid for each volume of NaOH added, e.g. when you enter 0.2, a total of 0.2 equivalents would have been added to the starting solution.

After obtaining the plot, please answer the following questions:

1. How many equivalents did you have to add to complete the titration? what is the relationship between the number of equivalents and the nature (e.g. monoprotic, diprotic) of the acid?
2. At what equivalent values is the pH of the solution equal to the pKa values of the acid?
3. At what pH values is the slope (d pH/d NaOH) of the curve the smallest?

An equivalent is simply the ratio of moles of titrant to moles of the weak acid. For example, if 0.5 equivalents of NaOH have been added to the solution then the moles of NaOH = 0.5 x moles of the weak acid.