How to Find the PKA of a Weak Acid

Calculating pKa is one way to identify an acid.
••• Jack Hollingsworth/Photodisc/Getty Images

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.

    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.

    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.

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

Related Articles

How to Calculate Kb From Ka
What pH Levels Are Considered Strong & Weak?
How to Use Baking Soda to Neutralize HCL
How to Determine pH From pKa
How to Calculate Titratable Acidity
How to Calculate the pH of Ammonia Water Using KB
How to Calculate End Point
How to Calculate Buffers
How to Calculate pKa Values
How to Find the Half Equivalence Point in a Titration...
How to Calculate Kb From Ka
How to Calculate the pH of a Strong Acid
How to Do Titration Calculations
How to Calculate Total Alkalinity
How to Calculate Ka From Ph
How to Determine Conjugate Bases of Acids
How to Neutralize an Acid
How to Calculate Kf
Sulfuric Acid & Chlorine Bleach Reaction
How to Find pH for a Given Molarity