QUESTION 132 (6 marks)
A 5.105 g sample of a pure monoprotic acid standard (molar mass 204.2 g mol⁻¹) is dissolved and made up to 250.0 mL. Separate 25.00 mL aliquots are titrated with NaOH. The titres are shown. A 25.00 mL aliquot of another monoprotic acid requires 18.80 mL of the same NaOH.
| Titration | Titre / mL |
|---|---|
| Rough | 20.80 |
| 1 | 19.95 |
| 2 | 20.00 |
| 3 | 20.05 |
(a) Use the three accurate titres to calculate the NaOH concentration. [3 marks]
(b) Calculate the concentration of the second acid. [1 marks]
(c) For the second acid, predict the effect on its calculated concentration if the endpoint is consistently overshot. Explain. [2 marks]
Practice marking scheme
Answer
The mean accurate titre is 20.00 mL; c(NaOH) = 0.1250 mol L⁻¹. The second acid concentration is 0.09400 mol L⁻¹. Overshooting makes the calculated acid concentration too high.
Working
The acid standard contains mol; each aliquot contains 0.002500 mol. Exclude the rough titre and average 19.95, 20.00 and 20.05 mL to obtain 20.00 mL. Thus . The second acid concentration is . An excessive titre is treated as if all added base neutralised acid, overestimating acid amount and concentration.
Marking criteria
- Uses the mean accurate titre 20.00 mL, excluding the rough titre. [1 mark]
- Calculates 0.002500 mol acid in each standard aliquot. [1 mark]
- Calculates c(NaOH) = 0.1250 mol L⁻¹. [1 mark]
- Calculates second acid concentration = 0.09400 mol L⁻¹. [1 mark]
- Predicts an overestimate of the acid concentration after overshooting. [1 mark]
- Explains that excess added titrant is incorrectly counted as acid-neutralising base. [1 mark]
Practice question aligned to the current QCAA syllabus; review the worked solution and marking criteria.
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