Question detail

What is the effect of adding a strong acid to a buffer solution?

Try the question, check the answer, then read the explanation to understand the curriculum point.

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At a glance

MCQ

Type

practice

Style

Topic

Acids and bases (A-level only)

Question

A. The pH changes only slightly
B. The pH remains constant
C. The pH increases significantly
D. The pH decreases significantly

Answer

The pH changes only slightly

Explanation

The correct option is The pH changes only slightly. The pH changes only slightly is the best answer because it directly supports the AQA A-Level Chemistry objective to sketch and explain pH curves for combinations of weak and strong monoprotic acids and bases. This reasoning is anchored to pH curves, titrations and indicators (A-level only) in Acids and bases (A-level only), and it separates pH curve from similar A-Level ideas rather than relying on a vague recall statement. Other options are weaker if they use the wrong evidence, calculation, mechanism, observation, unit, or conclusion for this subtopic.

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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