how to find ka from pka

What is pKa?

July 25, 2021

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.

Topics Covered in Other Articles

Acid-Base Chemistry
Strong Acids and Bases
Types of Chemical Reactions
Properties of Acids and Bases

Vocabulary

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

What is pKa?

In simple terms, pKa is a number that shows how weak or strong an acid is. A strong acid will have a pKa of less than zero. More precisely – pKa is the negative log base ten of the Ka value (acid 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.

Ka to pKa

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

pKa = -log[Ka]

pKa to 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 Weak & Strong Acids:

Hydrocyanic acid pKa = 9.21 (HCN, weak acid):

Acetic acid pKa = 4.75 (weak acid)

Hydrofluoric acid pKa = 3.14 (HF, weak acid)

Hydrochloric acid pKa = -8 (HCl, strong acid):

Sulfuric acid pKa ~ 3 (strong acid)

Some more pKa's are here.