Effect of Hydrogen Ion Concentration on pH (OCR GCSE Chemistry 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