Calculation

p-Functions ××=× 1 g m 000 g 1 35 453 mole Cl gCl. . 705 10 M Sometimes it is inconvenient to use the concentration units in Table 2.4. For example, during a reaction a reactant’s concentration may change by many orders of magnitude. If we are interested in viewing the progress of the reaction graphically, we might wish to plot the reactant’s concentration as a function of time or as a function of the volume of a reagent being added to the reaction. Such is the case in Figure 2.1, where the molar concentration of H+ is plotted (y-axis on left side of figure) as a function of the volume of NaOH added to a solution of HCl. The initial [H+] is 0.10 M, and its concentration after adding 75 mL of NaOH is 5.0 ´ 10–13 M. We can easily follow changes in the [H+] over the first 14 additions of NaOH. For the last ten additions of NaOH, however, changes in the [H+] are too small to be seen. When working with concentrations that span many orders of magnitude, it is often more convenient to express the concentration as a p-function. The p-function of a number X is written as pX and is defined as