pI is basically the pH at which the the charge of the compound is canceled out (meaning there is no net charge). In order to solve for it basically if you have lets say a amino acid with a nonpolar R group then we know the amino group is protonated at pH below 10 and we know the carboxyl group is protonated at pH below 2. So the PI would be an average of these two pka values and would thus be 6. Solving for the pI for amino acids with polar R groups is a little tricky....basically in this case you would take the average pka value of the two groups that would have the same charge in acid or base form.
