Titration Decision: Which Indicator and What Concentration?
A laboratory receives an unlabelled bottle said to contain hydrochloric acid. A 25.0 mL sample is titrated with a sodium hydroxide solution whose concentration was standardised at 0.100 mol/L. The burette reading starts at 0.50 mL and the endpoint is reached at 22.30 mL. The analyst uses phenolphthalein. A second analyst repeats the titration but stops at the first permanent pink colour that fades within 30 seconds, recording 21.90 mL. The true acid is later suspected to be a mixture of HCl and a weak acid.
1. Calculate the HCl concentration from the first titration and assess whether the result is reliable.
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Step 1 - Volume delivered. 22.30 − 0.50 = 21.80 mL = 0.02180 L.
Step 2 - Moles NaOH. 0.100 × 0.02180 = 2.180 × 10⁻³ mol.
Step 3 - Stoichiometry. HCl + NaOH → NaCl + H₂O, 1:1, so moles HCl = 2.180 × 10⁻³ mol.
Step 4 - Concentration. c = 2.180 × 10⁻³ / 0.0250 = 0.0872 mol/L.
Judgement: The arithmetic is sound, but reliability depends on the endpoint being the equivalence point. Phenolphthalein changes near pH 8–10, appropriate for strong acid–strong base. The result is reliable only if the acid is truly strong.
2. Compare the two analysts' endpoints and explain which is more trustworthy.
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Comparison: Analyst 1 delivered 21.80 mL; Analyst 2 delivered 21.40 mL. The difference is 0.40 mL, about 1.8 % of the titre.
Judgement: Analyst 2 stopped at a transient pink that faded, which indicates a premature endpoint - not enough base had been added for a stable colour. Analyst 1's persistent endpoint is closer to the true equivalence point. However, a single titration is weak evidence; the correct practice is concordant replicates (typically within 0.10 mL). The 0.40 mL spread signals technique error, so neither value should be reported without repeats.
3. If the sample is actually a mixture of HCl and a weak acid, how does that change the interpretation of the phenolphthalein endpoint?
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Reasoning: Phenolphthalein signals the point where the solution becomes basic enough (roughly pH 8–10). For a strong acid, that coincides with neutralising all HCl. For a weak acid, the equivalence point lies above pH 7 because the conjugate base hydrolyses; phenolphthalein may still be acceptable, but it measures total acid, not HCl alone.
Judgement: The calculated 0.0872 mol/L would represent total titratable acidity, not the HCl concentration. To separate the components, the analyst would need a second indicator or a pH curve showing two distinct steps. Reporting the value as 'HCl concentration' would be an over-interpretation of a single endpoint.