Choosing an Indicator and Predicting the Endpoint for a Weak Acid Titration
A student must determine the concentration of a 25.0 mL sample of ethanoic acid (Ka = 1.8 × 10⁻⁵) using 0.100 mol/L NaOH. The available indicators are methyl orange (pH 3.1–4.4), bromothymol blue (pH 6.0–7.6) and phenolphthalein (pH 8.2–10.0). A preliminary rough titration gave 23.5 mL; two accurate titrations gave 24.10 mL and 24.15 mL. The student notices that near the endpoint the pink colour fades on standing for about 30 seconds before becoming permanent.
1. Calculate the acid concentration and select the correct indicator, justifying the choice with a pH calculation at the equivalence point.
Read the worked answer
Concentration: mean titre = (24.10 + 24.15)/2 = 24.125 mL (the 23.5 mL rough value is discarded).
n(NaOH) = 0.100 × 0.024125 = 2.4125 × 10⁻³ mol = n(CH₃COOH).
c(acid) = 2.4125 × 10⁻³ / 0.0250 = 0.0965 mol/L.
Equivalence-point pH: total volume ≈ 25.0 + 24.1 = 49.1 mL; n(salt) = 2.41 × 10⁻³ mol, so [CH₃COO⁻] ≈ 2.41 × 10⁻³/0.0491 = 0.0491 mol/L.
For the conjugate base: Kb = Kw/Ka = 1.0 × 10⁻¹⁴/1.8 × 10⁻⁵ = 5.6 × 10⁻¹⁰.
[OH⁻] = √(Kb·c) = √(5.6 × 10⁻¹⁰ × 0.0491) = √(2.75 × 10⁻¹¹) = 5.2 × 10⁻⁶ mol/L → pOH = 5.28 → pH ≈ 8.7.
Indicator: the equivalence pH (≈8.7) lies inside the phenolphthalein range (8.2–10.0) and far outside methyl orange. Phenolphthalein is correct; methyl orange would change ~4 pH units before equivalence and give a large negative error.
2. Explain the fading pink colour and state whether the reported titre is likely to be high, low or correct, and how to fix the procedure.
Read the worked answer
Near the endpoint the solution is a buffer of unreacted acid and its salt, so added OH⁻ is partly consumed by residual CH₃COOH: CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O. The momentary pink is due to local excess of base; as it diffuses and is neutralised by remaining acid, the colour fades. This is normal and indicates the true endpoint has not yet been reached.
The correct action is to continue adding dropwise until a faint pink that persists for ~30 s is obtained; swirling between drops is essential. If the student instead recorded the first fleeting pink, the titre would be low (acid concentration underestimated). If the student overshot to a strong magenta, the titre would be high.
The fading itself does not bias the result if the student waits for persistence; the procedure is sound. Using a white tile and adding the last drops as fractions of a drop improves precision.
3. The student used a wet (rinsed with water, not dried) conical flask. Quantify the effect on the calculated acid concentration.
Read the worked answer
Residual water in the conical flask dilutes the acid sample but does not change the number of moles of CH₃COOH present. Since n(acid) = n(NaOH) at the endpoint, the titre is unchanged and the calculated concentration (which uses the fixed 25.0 mL aliquot volume and the titre) is unaffected.
This is the key distinction: water in the burette would dilute the NaOH and increase the titre (overestimating the acid); water in the conical flask is harmless. Rinsing the conical flask with the acid solution would add extra acid and increase the titre; rinsing with water is the safe practice.