pH Calculator


The pH calculator aims to determine the pH value of an aqueous solution of a given compound, or the molar concentration of a specific pH value solution. To prepare a solution with a specific molar concentration based on mass, use Mass Molarity Calculator. To prepare a solution with a specific molar concentration based on concentration, use Solution Dilution Calculator.

mol/L =

Sometimes it is inconvenient to use the concentration units. For example, 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 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 mol/L, and its concentration after adding 75 mL of NaOH is 5.0 × 10–13 mol/L. 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

Figure 1. Graph of [H+] versus volume of NaOH and pH versus volume of NaOH for the reaction of 0.10 mol/L HCl with 0.10 mol/L NaOH.

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

pX = –lg(X)

Thus, the pH of a solution that is 0.10 mol/L H+ is

pH = –lg [H+] = –lg(0.10) = 1.00

and the pH of 5.0 × 10–13 mol/L H+ is

pH = –lg[H+] = –lg(5.0 × 10–13) = 12.30

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