Effect of Hydrogen Ion Concentration on pH (OCR GCSE Combined Science A (Gateway)): Revision Note
Effect of Hydrogen Ion Concentration on pH
Higher Tier Only
pH is a measure of the concentration of H+ ions in solution
A concentrated solution of either an acid or a base is one that contains a high number of acid or base molecules per dm3 of solution so would produce pH values below 4 and above 10
A dilute acid or base solution is therefore one that has much fewer acid or base molecules per dm3 of solution, hence the pH value would lie between 5 and 9
The pH scale is logarithmic, meaning that each change of 1 on the scale represents a change in concentration by a factor of 10
Therefore an acid with a pH of 3 has ten times the concentration of H+ ions than an acid of pH 4
An acid with a pH of 2 has 10 x 10 = 100 times the concentration of H+ ions than an acid with a pH of 4
From this we can summarize that for two acids of equal concentration, where one is strong and the other is weak, the strong acid will have a lower pH due to its capacity to dissociate more and hence put more H+ ions into solution than the weak acid
Titration Curves
A pH titration curve shows the effect of changing hydrogen ion concentration on pH
The titration curve on the left demonstrates what happens to the pH when a base is added to an acid
The pH increases rapidly due to a decrease in the concentration of hydrogen ions
The titration curve on the right demonstrates what happens to pH when an acid is added to a base
The pH decreases rapidly due to an increase in the concentration of hydrogen ions

Titration curves demonstrating the effect of changing pH when a base is added to an acid and vice versa
Examiner Tips and Tricks
Acid strength indicates the proportion of acid molecules that dissociate while concentration is a measure of how much acid there is per unit volume of water.
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