9701 · 25.1
Acids and bases — FAQ
Frequently asked questions for 9701 Acids and bases. Direct answers first, then deeper explanation — then practise with marking.
What is the difference between a strong acid and a concentrated acid?
Strength and concentration are different concepts. 'Strength' refers to the degree of dissociation of an acid in water; a strong acid like HCl dissociates completely. 'Concentration' refers to the amount of acid substance dissolved in a unit volume of solution. It is possible to have a dilute solution of a strong acid (e.g., 0.01 M HCl) or a concentrated solution of a weak acid (e.g., 17 M ethanoic acid).
Why is the pH at the equivalence point not always 7?
The pH at the equivalence point is 7 only for a strong acid-strong base titration. For a weak acid-strong base titration, the salt formed at equivalence contains the conjugate base of the weak acid, which hydrolyses water to produce OH⁻ ions, making the solution alkaline (pH > 7). Conversely, for a strong acid-weak base titration, the conjugate acid formed makes the solution acidic (pH < 7).
How do I choose the correct indicator for a titration?
The indicator's pKin value (the pH at which it changes colour) should be as close as possible to the pH at the equivalence point of the titration. More accurately, the entire pH range of the indicator's colour change must lie on the steep, vertical part of the titration curve. For example, phenolphthalein (pH range 8.3-10.0) is suitable for weak acid-strong base titrations, while methyl orange (pH range 3.1-4.4) is suitable for strong acid-weak base titrations.