An approach is presented that utilizes a spreadsheet to allow students to explore different means of calculating and visualizing how the charge on peptides and proteins varies as a function of pH. In particular, the concept of isoelectric point is developed to allow students to compare the results of their spreadsheet calculations with those of computer programs that are freely available on the Internet and are designed to calculate the same parameter. Such comparisons allow students to explore the underlying assumptions of their model (and the likely assumptions of the computer programs) and to speculate on ways in which their model might be improved. A discussion of the oligomeric protein hemoglobin is included to show the limitations of the computer programs and to show how the spreadsheet approach can be adapted to treat satisfactorily this important protein. (Contains 2 tables, 2 figures and 1 scheme.)