Choosing a Method to Find the Concentration of Vinegar
A student is given a bottle of clear white vinegar labelled '5% acidity' and must check the claim using school apparatus. Available: a 25 cm3 pipette, a burette, conical flasks, a balance reading to 0.01 g, a 250 cm3 volumetric flask, phenolphthalein, and a standard 0.100 mol/dm3 sodium hydroxide solution. The student first tries titrating the vinegar directly and finds the burette reading changes by about 24 cm3, but the end-point fades and reappears. A second student suggests instead weighing 10.00 g of vinegar, diluting it to 250 cm3, and titrating 25.0 cm3 portions. The teacher asks which method is more reliable and how the result should be interpreted.
1. Explain why the direct titration gave a fading end-point and why dilution improves the procedure.
Read the worked answer
Vinegar contains ethanoic acid, a weak acid. During direct titration the acid is concentrated, so near the end-point the reaction is slow and local excesses of alkali are neutralised only gradually; the indicator colour therefore fades as equilibrium re-establishes. Diluting the vinegar lowers the acid concentration, so each addition of NaOH produces a sharper, more stable colour change and the end-point is easier to judge reproducibly. Dilution also brings the titre into a convenient burette range. The number of moles of acid in the weighed sample is unchanged by dilution, so the calculation remains valid.
2. The diluted titration used 21.40 cm3 of 0.100 mol/dm3 NaOH per 25.0 cm3 portion. Calculate the mass of ethanoic acid in the original 10.00 g sample and comment on the 5% claim.
Read the worked answer
Moles NaOH = 0.100 × 21.40/1000 = 2.140 × 10^-3 mol. Since CH3COOH + NaOH → CH3COONa + H2O, moles acid in the 25.0 cm3 portion = 2.140 × 10^-3 mol. The 250 cm3 flask contains 10 times this: 2.140 × 10^-2 mol. Mass = 2.140 × 10^-2 × 60.0 = 1.284 g. Percentage by mass = 1.284/10.00 × 100 = 12.8%. This is far above 5%, so either the label is wrong or, more likely, the student mis-recorded the dilution or titre. The result should be checked before being reported; a 5% vinegar would need only about 8.3 cm3 of the same NaOH per portion.
3. State two precautions that would improve the reliability of the result and explain each briefly.
Read the worked answer
Rinse the pipette with the diluted vinegar before use so that residual water does not dilute the portion; this keeps the moles delivered exactly known. Rinse the burette with the NaOH solution so that residual water does not lower the alkali concentration. Read the burette at eye level at the bottom of the meniscus to avoid parallax error, and repeat titrations until concordant results (within 0.10 cm3) are obtained, using the average of the concordant titres.