More about acids - Higher

Dilute and concentrated solutions

A solution forms when a solute dissolves in a solvent. The more concentrated the solution, the more particles it contains in a given volume.

When solutions are described as dilute or concentrated:

  • a dilute solution contains a relatively small amount of dissolved solute in a given volume
  • a concentrated solution contains a relatively large amount of dissolved solute in a given volume

Take care to use the word ‘dilute’ correctly. It can be used as an adjective to describe the concentration of a solution (as here), or as a verb to describe the process of adding more water to a solution to reduce its concentration.

Strong and weak acids

Acids in solution are a source of hydrogen ions, H+. The hydrogen ions are produced when the acid dissociates or breaks down to form ions.

Strong acids

Strong acids completely dissociate into ions in solution. For example, hydrochloric acid is a strong acid. It completely dissociates to form hydrogen ions and chloride ions:

HCl(aq) → H+(aq) + Cl-(aq)

Nitric acid and sulfuric acid are also strong acids.

Weak acids

Weak acids only partially dissociate into ions in solution. For example, ethanoic acid is a weak acid. It only partially dissociates to form hydrogen ions and ethanoate ions:

CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq)

The ⇌ symbol is used in the equation to show that the reaction is a reversible reaction and does not go to completion.

curriculum-key-fact
A strong acid has a higher pH than a weak acid of the same concentration.

pH and hydrogen ion concentration

The pH of a solution is a measure of its concentration of hydrogen ions. The higher the concentration of H+ ions in an acidic solution, the lower the pH.

A pH of 1 represents a hydrogen ion concentration of 0.1 mol/dm3.

ConcentrationpH
1 mol/dm30
0.1 mol/dm31
0.01 mol/dm32
0.001 mol/dm33
0.0001 mol/dm34

If the hydrogen ion concentration in a solution increases by a factor of 10, the pH of the solution decreases by 1.

If the hydrogen ion concentration increases by a factor of 100 (102), the pH decreases by 2.

pH of alkaline solutions

The higher the concentration of OH- ions in an alkaline solution, the higher the pH.

Example

A solution of 0.8 mol/dm3 hydrochloric acid has a pH of 0.1. Predict its pH when it is diluted to 0.08 mol/dm3.

The hydrogen ion concentration decreases by a factor of 10, so the pH increases by 1 from 0.1 to 1.1.

Question

A solution of 0.5 mol/dm3 hydrochloric acid has a pH of 0.3. Predict its pH when it is diluted to 0.005 mol/dm3.

The hydrogen ion concentration decreases by a factor of 100 (or 102), so the pH increases by 2 from 0.3 to 2.3.

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