Tutorials

What is pKa?

Core Concepts

In this tutorial, you will learn about pKa and its relationships with Ka and pH as well as how to calculate pKa from Ka and pH.

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Vocabulary

Bronsted Acid – any species capable of donating a proton (H+) to another molecule.

What is pKa?

pKa denotes the logarithmic dissociation constant. It measures the strength of an acid — how tightly a proton is held by a Bronsted acid. The lower the value of pKa, the stronger the acid and the greater its ability to donate its protons.

pKa and Ka

Ka denotes the acid dissociation constant. It measures how completely an acid dissociates in an aqueous solution. The larger the value of Ka, the stronger the acid as acid largely dissociates into its ions.

The relationship between pKa and Ka is described by the following equation:

pKa = -log[Ka]

Conversely, Ka is measurable when pKa is given:

Ka = 10-pKa

pKa and pH

pH is a measure of the concentration of hydrogen ions in an aqueous solution. The lower the pH value, the higher the hydrogen ion concentration in the solution; therefore, the stronger the acid.

pKa and pH are related as pKa helps predict what a molecule will do at a specific pH. Essentially, pKa reveals what the pH needs to be in order for a chemical species to be able to donate or accept a proton.

The relationship between pKa and pH is described by the Henderson-Hasselbalch equation:

\text{pH}=\text{pKa}+\frac{\text{A}^{-}}{\text{HA}}

\text{pH}=\text{pKa}+\frac{\text{conjugate base}}{\text{weak acid}}

pKa of Some Acids:

Acetic acid: pKa = 4.75

Hydrochloric acid: pKa = -8

Sulfuric acid: pKa ~ 3

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