Chemistry 3.4, 3.5, 3.7, 3.8 - Concentration and acid strength

How ion concentration controls pH

Higher tier only

The pH scale is linked to the concentration of ions in the solution:

• in an acidic solution, a higher concentration of $\ce{H+}$ means a lower pH;
• in an alkaline solution, a higher concentration of $\ce{OH-}$ means a higher pH.

This explains the direction seen in the practical at the particle level. While the mixture is acidic, adding calcium hydroxide or calcium oxide causes the hydrogen-ion concentration to fall as the solid reacts with the acid, so the measured pH rises. Once the mixture is alkaline, hydroxide ions are present in excess and the pH is above 7.

The pH scale is not linear. For acidic solutions, every factor-of-10 increase in hydrogen-ion concentration makes the pH decrease by 1.

Worked example: a factor-of-1000 change

An acidic solution starts at pH 4.8. Its hydrogen-ion concentration increases by a factor of 1000.

1. Resolve the factor: $1000 = 10 \times 10 \times 10$, so there are three factor-of-10 increases.
2. Each factor-of-10 increase lowers pH by 1, so the total change is a decrease of 3 pH units.
3. Calculate: new pH = $4.8 - 3 = 1.8$.

The result is pH 1.8, with no unit. It is lower than 4.8, which is sensible because the hydrogen-ion concentration increased.

You do not need the logarithmic pH formula or pH calculations from concentrations for this specification point. The assessed relationship is the factor-of-10 rule.
The rule works in reverse: if hydrogen-ion concentration falls by a factor of 10, pH rises by 1. For example, a hundredfold decrease changes pH 2.4 to pH 4.4. This is a change in hydrogen-ion concentration, not evidence that the acid itself has changed from strong to weak.

Strength is not concentration

Higher tier only

Two independent questions describe an acid solution:

1. How much acid is present per volume of solution? This is concentration.
2. What fraction of the dissolved acid particles dissociates into ions? This is acid strength.

A concentrated solution contains a relatively large amount of dissolved substance in a given volume. A dilute solution contains a relatively small amount in that volume. Dilution changes the amount of acid per volume by adding solvent; it does not turn one type of acid into a different type.

A strong acid has a very high degree of dissociation: all or nearly all of its dissolved acid particles form ions in solution. A weak acid dissociates only partially, so many acid particles remain undissociated. "Weak" does not mean harmless, and "strong" does not mean concentrated.

Imagine 100 dissolved acid particles as a model. For a strong acid, nearly all would be represented as separated ions. For a weak acid, only some would be represented as ions and many would remain as acid particles. The number 100 is only a comparison model; it is not a fixed percentage for every acid.