Titrating an Unknown Acid with Limited Indicator
A student receives 25.0 mL of a clear, colorless solution labeled only 'monoprotic acid, concentration unknown.' She has a 0.100 M NaOH solution, a burette, and three indicators: phenolphthalein (color change pH 8.2–10.0), methyl orange (pH 3.1–4.4), and bromothymol blue (pH 6.0–7.6). She titrates three 25.0 mL samples and records endpoint volumes of 22.4 mL, 22.6 mL, and 22.5 mL using phenolphthalein. A classmate argues she should have used methyl orange because 'acids turn methyl orange red.'
1. Which indicator is appropriate for this titration, and why is the classmate's reasoning flawed?
Read the worked answer
Use phenolphthalein. For a strong base (NaOH) titrating an acid, the equivalence point is near pH 7 or above; phenolphthalein's range (8.2–10.0) brackets the steep part of the curve on the basic side, giving a sharp, reliable endpoint.
Methyl orange changes at pH 3.1–4.4, which is reached long before neutralization is complete, so it would signal the endpoint too early and underestimate the acid. The classmate confuses 'acid turns methyl orange red' (a qualitative color fact) with 'the endpoint of this titration occurs in methyl orange's range' (a pH-matching requirement). Indicator choice depends on where the pH jumps, not on the color of the starting solution.
2. Compute the acid concentration from the data and state the uncertainty in the volume.
Read the worked answer
Mean volume = (22.4 + 22.6 + 22.5) / 3 = 67.5 / 3 = 22.5 mL.
Moles NaOH = 0.100 mol/L × 0.0225 L = 2.25 × 10⁻³ mol.
Monoprotic acid: moles acid = moles base = 2.25 × 10⁻³ mol.
Concentration = 2.25 × 10⁻³ mol ÷ 0.0250 L = 0.0900 M.
Spread of readings is 22.4–22.6 mL, i.e. ±0.1 mL, about ±0.4%. So report ≈ 0.090 M (roughly 0.0896–0.0904 M). The three trials agree closely, which supports the phenolphthalein endpoint.
3. Suppose the acid were actually weak (e.g., acetic acid). How would the equivalence-point pH shift, and would phenolphthalein still be acceptable?
Read the worked answer
For a weak monoprotic acid titrated with strong base, the salt formed is basic (conjugate base hydrolyzes), so the equivalence point lies above pH 7, typically pH 8–9 for something like acetate at these concentrations.
Phenolphthalein (8.2–10.0) still covers this region, so it remains a good choice - arguably better than for a strong acid. Methyl orange (3.1–4.4) would be clearly wrong because the steep jump is on the basic side. Judgement point: the student cannot know if the acid is strong or weak from appearance alone, but phenolphthalein works for both cases here, so her choice is robust.