Concentration and strength describe different things
Definition
Concentration
The mass or amount of a solute dissolved in a given volume of solution.
Definition
Dissociation
The separation of a substance into ions when it dissolves in water.
- Concentration describes how much acid is dissolved in a given volume of solution.
- Strength describes what proportion of the dissolved acid molecules split up into ions.
- The two are independent, so an acid can be dilute and strong, or concentrated and weak.
- Adding water changes the concentration but never the strength.
- Strength is a property of the acid itself, fixed by which acid it is.
Key Idea
- Concentration is how much acid there is in a given volume.
- Strength is what proportion of it dissociates, and that is fixed by which acid it is rather than by how much water is present.
Dilute and concentrated describe how much acid is present
Definition
Dilute solution
A solution containing a relatively small amount of solute in a given volume.
Definition
Concentrated solution
A solution containing a relatively large amount of solute in a given volume.
- A dilute acid holds a relatively small amount of acid in a given volume of solution.
- A concentrated acid holds a relatively large amount in the same volume.
- Adding water increases the volume without changing the amount of acid, so the solution becomes more dilute.
- Removing water reduces the volume, so the solution becomes more concentrated.
- Diluting an acid raises its pH, because the H+\text{H}^{+}H+ concentration falls.
Example
- Diluting tenfold: the H+\text{H}^{+}H+ concentration falls by a factor of 101010, so the pH rises by 111.
- Two bottles of one acid: one labelled dilute and one concentrated hold the same acid at different concentrations.
Strong and weak describe how far the acid dissociates
Definition
Strong acid
An acid whose molecules dissociate completely into ions in aqueous solution.
Definition
Weak acid
An acid whose molecules dissociate only partially into ions in aqueous solution.
- A strong acid dissociates completely, so every molecule splits into ions in solution.
- Hydrochloric acid is strong: HCl(aq)→H+(aq)+Cl−(aq)\text{HCl}(aq) \rightarrow \text{H}^{+}(aq) + \text{Cl}^{-}(aq)HCl(aq)→H+(aq)+Cl−(aq).
- A weak acid dissociates only partially, so most of it stays as whole molecules.
- Ethanoic acid is weak: CH3COOH(aq)⇌H+(aq)+CH3COO−(aq)\text{CH}_3\text{COOH}(aq) \rightleftharpoons \text{H}^{+}(aq) + \text{CH}_3\text{COO}^{-}(aq)CH3COOH(aq)⇌H+(aq)+CH3COO−(aq).
- The reversible arrow shows that ions and undissociated molecules are both present at once.
- At the same concentration a strong acid gives a higher H+\text{H}^{+}H+ concentration than a weak one.
- The strong acid therefore has the lower pH of the two at equal concentration.
Common Mistake
- Strong is not concentrated, because strength is about dissociation and concentration about amount.
- Weak is not dilute, for exactly the same reason.
Describing an acid needs both words
- Dilute and strong: little acid present, but all of it dissociated.
- Concentrated and strong: a lot of acid present, all of it dissociated.
- Dilute and weak: little acid present, and only some of it dissociated.
- Concentrated and weak: a lot of acid present, but only some of it dissociated.
- Give the two descriptions separately, because they answer different questions.
- A concentrated weak acid can hold a higher H+\text{H}^{+}H+ concentration than a dilute strong one.
Exam technique
- Keeping concentration for amount and strength for dissociation stops the two ideas blurring together.
- A comparison of pH only holds at the same concentration, so that condition is worth stating.
- The reversible arrow is the mark of a weak acid, and it belongs in the equation.
Self review
- What does the concentration of an acid describe?
- What happens to the concentration, and to the strength, when water is added?
- What is the difference between complete and partial dissociation?
- Why is hydrochloric acid described as a strong acid?
- Can an acid be both concentrated and weak?
