Students should be able to: sketch and explain the shapes of typical pH…

The idea

A pH curve plots pH against volume of titrant added. Its shape depends on the strengths of the acid and base. A strong acid starts at a low pH (e.g. pH 1), while a weak acid starts higher (e.g. pH 3) and shows a buffer region. The equivalence point is marked by a near-vertical steep section. For strong acid-strong base, this spans pH 3-11; for weak acid-strong base, pH 7-11; for strong acid-weak base, pH 3-7. Weak acid-weak base curves lack a steep section. An indicator is a weak acid whose two forms are different colours. It is usable only if its colour change range (roughly pKIn ± 1) falls entirely within the steep section of the pH curve. Methyl orange (pH 3.1-4.4) suits strong acid-weak base; phenolphthalein (pH 8.3-10.0) suits weak acid-strong base.

What earns the marks

• Sketch a pH curve with the correct initial pH and final pH for the given acid and base.
• Draw the steep section at the correct equivalence volume and pH range, noting that strong acid-strong base spans pH 3-11.
• State that the indicator's pH range must lie entirely within the steep section of the curve.
• Choose phenolphthalein for a weak acid with a strong base, as its colour change range falls within the steep section of pH 7-11.

Common mistakes and how to put them right

• Drawing the steep section of a weak acid-strong base curve extending down to pH 3 instead of stopping around pH 7.

• Choosing an indicator because its range straddles pH 7 rather than because it lies entirely within the steep section.

• Using phenolphthalein for ammonia titrated with hydrochloric acid, where it changes long before the equivalence point.

Exam tips

• When sketching a pH curve, clearly mark the initial pH, the equivalence point volume, the steep section range, and the final pH.
• Quote the indicator range and the span of the steep section side by side; the comparison is the explanation.