Question detail
When titrating a weak acid with a strong base, what is observed in the pH curve?
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Acids and bases (A-level only)
Question
- A. A gradual increase in pH with a sharp rise at the equivalence point
- B. A steep decline in pH throughout the titration
- C. A flat curve with no significant changes
- D. A gradual decrease in pH followed by a plateau
Answer
A gradual increase in pH with a sharp rise at the equivalence point
Explanation
In the titration of a weak acid with a strong base, the pH increases gradually as the base is added, but there is a sharp rise at the equivalence point due to the complete neutralization of the weak acid, leading to a rapid change in pH.
Common mistake
Misunderstanding pH Curve Shapes
Students often incorrectly assume that the pH curve for a weak acid-strong base titration will have a steep slope throughout the entire range, similar to that of a strong acid-strong base titration.
To correct this, remember that the pH curve for a weak acid-strong base titration will show a more gradual slope before the equivalence point due to the weak acid's partial dissociation. The formula for pH at the equivalence point can be derived from the concentration of the conjugate base formed. For example, if you have a weak acid HA with a concentration of 0.1 mol/dm³ and a Ka of 1.8 x 10^-5, you can calculate the pH at the equivalence point using the formula: pH = 14 - 0.5 * pKa, where pKa = -log(Ka). Substituting gives pH = 14 - 0.5 * 4.74 = 11.63. Therefore, the pH at the equivalence point is 11.63.
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