How To Find The PKA Of A Weak Acid

The pKa of an acid is a constant associated with how much the acid ionizes in solution. pKa is a combination of the p in pH, which stands for power of hydrogen, and the dissociation constant for acids, represented by Ka. Since strong acids, by definition, ionize completely, pKa is more important as a characteristic of weak acids. You can use it to predict an acid's pH if you know the concentration. The easiest way to find the pKa of an acid is to refer to a table. However in some cases, the type of acid may be unknown, so calculating the pKa can help you identify the acid. The calculation is straightforward if you know the solution's concentration and pH.

Step 1

Calculate the concentration of hydrogen ions with the formula [H+] = 1/(10^pH). The units for concentration are moles per liter, where a mole is a set of particles whose quantity equals 6.02 x 10^23. For example, if the pH of the solution is 2.29, the concentration is [H+] = 1/(10^2.29) = 5.13 x 10^-3 moles/liter.

Step 2

Find the dissociation constant with the formula Ka = [H+]²/([HA] – [H+]), where Ka is the dissociation constant, and [HA] is the concentration of the acid before dissociation. For example, if the initial concentration is 0.15 moles/liter, Ka = (5.13 x 10^-3)²/(0.15 – (5.13 x 10^-3)) = 1.82 x 10^-4.

Step 3

Calculate the pKa with the formula pKa = -log(Ka). For example, pKa = -log(1.82 x 10^-4) = 3.74.