In this tutorial, you will learn about pKa and its relationships with Ka and pH as well as how to calculate it from Ka and pH.
* Before reading this article it is imperative to have some common knowledge about pH and what pH is. It is recommended to read this article (What is pH? The Formula & Equation) to refresh yourself on the terms discussed in this tutorial.
Topics Covered in Other Articles
- Acid-Base Chemistry
- Strong Acids and Bases
- Types of Chemical Reactions
- Properties of Acids and Bases
- Buffers in Biological Systems
Bronsted Acid – any species capable of donating a proton (H+) to another molecule.
Strong acid- A strong acid is one that is completely dissociated or ionized in an aqueous solution.
pKa- a number that shows how strong or weak an acid is.
Ka – The acid dissociation constant.
Base – A substance that can neutralize the acid by reacting with hydrogen ions
What is pKa?
It is a quantitative way to measure the behavior of acids and bases in a solution. Usually the pH of a solution can be predicted if the concentration and pKa values of all acids and bases are known.
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 a negative value 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, the stronger the acid and the greater its ability to donate its protons.
The Values relation to 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 following equation describes the relationship between pKa and Ka:
pKa = -log[Ka]
Ka symbolizes the value of the acid dissociation constant. The purpose of Ka is to measure how much an acid will dissociates in an aqueous solution. Strong acids tend to completely dissolve in solutions, so the larger the Ka value the stronger the acid.
Conversion 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 it helps predict what a molecule will do at a specific pH. Essentially, the value 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:
pH = pKa + log ([conjugate base]/[weak acid])
pH = pka+log ([A–]/[HA])
pKa and Buffers
While this is commonly used to determine the strength of an acid, it can also be used to select buffers. This is possible because of the relationship between pKa and pH:
pH = pKa + log10([A–]/[AH])
The equation shows that pKa and ph are equal to each other when only half of the acid has dissociated. The ability of a buffer to maintain the pH of a solution is highest when the two values are close. If you ever have to select a buffer, the probable choice is one that has a value close to the target pH of the chemical solution.
pKa of Some Weak & Strong Acids:
Hydrocyanic acid = 9.21 (HCN, weak acid):
Acetic acid = 4.75 (weak acid)
Hydrofluoric acid = 3.14 (HF, weak acid)
Hydrochloric acid = -8 (HCl, strong acid):
Sulfuric acid ~ 3 (strong acid)
Some more pKa’s are here.
Our favorite Chemistry Experiment
No acids, based or pKA’s in this video, just some very cool chemistry!