In this tutorial, you will learn about the properties and pH of strong acids and bases, and how to calculate their pH, pOH, pKa, and pKb values.
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pH – a measure of the concentration of hydrogen ion in a solution.
pOH – a measure of the concentration of hydroxide ion in a solution.
Strong Acids – Introduction
A strong acid (HA) completely dissociates in a solution. In other words, it ionizes 100% in an aqueous solution. Moreover, strong acids are good proton donors and they can remain in aqueous solution. A strong proton donor is defined as a molecule in which a hydrogen is attached to an electronegative atom, such as oxygen or a halogen (fluorine, chlorine, bromine, iodine, astatine, and tennessine). In water, a strong acid loses a proton and transfers it to H2O molecules, forming H3O — a hydronium ion.
HA + H2O → H3O+(aq) + A–(aq)
In this example, A represents the anion of an acid, for example a chloride or nitrate ion.
Strong acids have a large value of Ka (acid dissociation constant) and a small value of pKa (logarithmic acid dissociation constant).
pH and pKa of a Strong Acid
Strong acids like hydrochloric acid (HCl) have a pH around 0 to 1. The lower the pH value, the higher the concentration of hydrogen ions in the solution, therefore, a stronger acid.
pH = -log [H+]
The acid dissociation constant, Ka, measures how completely an acid dissociates in an aqueous solution. The large Ka value indicates a strong acid, as it means the acid is largely dissociated into its ions. Since strong acids completely dissociate in a solution, they have a small value of Ka.
The logarithmic acid dissociation constant, pKa, has a direct relationship to Ka. Therefore, the lower the pKa value, the stronger the acid.
pKa = -log [Ka]
Conjugate Base of a Strong Acid
The conjugate base of an acid, is the ion that results when the acid loses a proton. For example, the conjugate base of HCl is the chloride ion, Cl–. The conjugate base of a strong acid is a very weak base. Salts formed from those conjugate bases often have a pH of 7, or just above 7. For example, sodium chloride, sodium sulfate, and sodium nitrate all have anions that are conjugate bases of strong acids. All of those salts have a pH near 7 in solution – which is a neutral pH.
A strong base (BOH) also completely dissociates and ionizes 100% in an aqueous solution. Moreover, strong bases are good proton acceptors, which cannot remain in aqueous solution. For instance, all O2- ions are converted to OH– ions by accepting protons from H2O molecules. As a result, H2O molecules are converted to OH–. It is important to note that the cations of the strong bases are soluble in water. In other words, if a cation is soluble is water, it can form a strong base.
BOH + H2O → B+(aq) + OH–(aq)
pOH and pKb of a Strong Base
The pOH is a measure of hydroxide ion (OH-) concentration. The lower the pOH value, the higher the concentration of hydroxide ions in the solution, therefore, a stronger base.
pOH = -log [OH–]
The base dissociation constant, Kb, measures how completely a base dissociates in an aqueous solution. The large Kb value indicates a strong base, as it means the base is largely dissociated into its ions.
With the Kb value, you can calculate pKb. The lower the pKb value, the stronger the base.
pKb = -log [Kb]
The pH and pOH of an aqueous solution are related by the following equation:
pH + pOH = 14
If either the pH or the pOH value of a solution is known, the other can be calculated. Strong bases generally have a pH between 13 and 14.
Calculating the pH of a Strong Acids and Bases Examples
Q1. Suppose you have a 0.1M solution of HCl. Calculate its pH.
HCl + H2O → H3O+(aq) + Cl–(aq)
Since hydrochloric acid is strong, we get 100% ionization. O.1M is also the concentration of hydronium ions.
To calculate the pH, plug in 0.1M into the pH equation:
pH = -log [H3O+] = -log (0.1) = 1
Q2. Suppose you have a 0.1M solution of NaOH. Calculate its pH.
NaOH + H2O → Na+(aq) + OH–(aq)
Since sodium hydroxide is a strong base, we get 100% ionization. O.1M is also the concentration of hydroxide ions.
To calculate the pH, plug in 0.1M into the pOH equation:
pOH = -log [OH–] = -log (0.1) = 1
Then, use the following equation to solve for pH:
pH + pOH = 14
pH = 14 – pOH = 14 – 1 = 13
List of Strong Acids and Bases
List of Strong Acids (7):
- HCl (hydrochloric acid)
- HNO3 (nitric acid)
- H2SO4 (sulfuric acid)
- HBr (hydrobromic acid)
- HI (hydroiodic acid)
- HClO3 (chloric acid)
- HClO4 (perchloric acid)
List of Strong Bases (8):
- LiOH (lithium hydroxide)
- NaOH (sodium hydroxide)
- KOH (potassium hydroxide)
- Ca(OH)2 (calcium hydroxide)
- RbOH (rubidium hydroxide)
- Sr(OH)2 (strontium hydroxide)
- CsOH (cesium hydroxide)
- Ba(OH)2 (barium hydroxide)
Did you know that fluorantimonic acid is an example of a very strong acid, known as a super acid? It is extremely corrosive and must be stored in teflon containers. A super acid has an extreme ability to force a proton onto another molecule.