Concentrations and strengths of acids - Higher
Dilute and concentrated solutions
A solution forms when a soluteThe dissolved substance in a solution. dissolveWhen a substance breaks up and mixes completely with a solvent to produce a solution. in a solventThe liquid in which the solute dissolves to form a solution.. The more concentrationThe concentration of a solution tells us how much of a substance is dissolved in water. The higher the concentration, the more particles of the substance are present. the solution, the more particleA general term for a small piece of matter. For example, protons, neutrons, electrons, atoms, ions or molecules. it contains in a given volumeThe volume of a three-dimensional shape is a measure of the amount of space or capacity it occupies, eg an average can of fizzy drink has a volume of 330 ml..
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 鈥榙ilute鈥 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
acidSubstance producing more hydrogen ions than hydroxide ions when dissolved in water. in solution are a source of hydrogen ionElectrically charged particle, formed when an atom or molecule gains or loses electrons., H+. The hydrogen ions are produced when the acid dissociationThe breaking up of a molecule into ions when dissolved in water. 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) 鈫 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) 鈬 H+(aq) + CH3COO-(aq)
The 鈬 symbol is used in the equation to show that the reaction is a reversible reactionA chemical reaction which can go both ways. and does not go to completion.
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.
| Concentration | pH |
| 1 mol/dm3 | 0 |
| 0.1 mol/dm3 | 1 |
| 0.01 mol/dm3 | 2 |
| 0.001 mol/dm3 | 3 |
| 0.0001 mol/dm3 | 4 |
| Concentration | 1 mol/dm3 |
|---|---|
| pH | 0 |
| Concentration | 0.1 mol/dm3 |
|---|---|
| pH | 1 |
| Concentration | 0.01 mol/dm3 |
|---|---|
| pH | 2 |
| Concentration | 0.001 mol/dm3 |
|---|---|
| pH | 3 |
| Concentration | 0.0001 mol/dm3 |
|---|---|
| pH | 4 |
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.