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S6 Chemistry: Instrumental Analysis (Paper 4: Misconception Drill)
S6 Chemistry: Instrumental Analysis (Paper 4: Misconception Drill)
Ten items targeting the commonest instrumental analysis misconceptions in infrared, mass spectra, colorimetry and chromatography.
Direct answers Answers in order: no, bromine, 0.25 mol dm-3, 20 times, no, a carbonyl, no because Rf is 0.50, no, multiply, not necessarily.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 20 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: S6 Chemistry: Instrumental Analysis |
| Level | S6 |
02 Questions (10 items)
- FoundationDrill: does an infrared spectrum show a molecular ion peak?
- FoundationDrill: equal height M and M+2 peaks indicate chlorine or bromine?
- FoundationDrill: a 0.10 mol dm-3 standard reads 0.20 and the sample reads 0.50. What is the sample concentration?
- IntermediateDrill: a 5.0 cm3 sample is made up to 100 cm3. What is the dilution factor?
- IntermediateDrill: is acid-base titration an instrumental method?
- IntermediateDrill: does a strong band at 1700 cm-1 mean a hydroxyl or a carbonyl?
- ChallengeDrill: a spot moves 3.0 cm while the solvent front moves 6.0 cm. Is Rf 2.0?
- ChallengeDrill: can concentration be found by dividing absorbance by the molar absorption coefficient alone?
- ChallengeDrill: after dilution the absorbance falls. To recover the original concentration, multiply or divide by the dilution factor?
- Error redoDrill: does an identical retention time prove two samples are the same compound?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | No, that only appears in a mass spectrum | Infrared records absorbed wavenumbers while mass spectrometry records mass to charge ratio. |
| 2 | Bromine | Chlorine gives M+2 at about one third of M, so nearly equal heights point to bromine. |
| 3 | 0.25 mol dm-3 | Absorbance is proportional to concentration and 0.50 is 2.5 times 0.20. |
| 4 | 20 times | The dilution factor is the final volume over the aliquot volume, 100 over 5.0. |
| 5 | No, it is classical volumetric analysis | Instrumental methods measure a physical quantity with an instrument, while titration uses an indicator and burette readings. |
| 6 | A carbonyl, C=O | A hydroxyl gives a broad band at 3200 to 3600, so the strong 1700 band is a carbonyl. |
| 7 | No, Rf is 3.0 over 6.0, that is 0.50 | Rf is always below 1, with the spot distance over the solvent front distance. |
| 8 | No, the path length must be divided out as well | Since A equals epsilon c l, c equals A divided by epsilon and then by the path length. |
| 9 | Multiply by the dilution factor | Dilution lowers the concentration, so recovering it requires multiplication. |
| 10 | Not necessarily, mass spectrometry or spiking with a standard is also needed | Different compounds can co-elute under the same conditions, so retention time alone is only a necessary condition. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
Turn every wrong item today into one sentence: I thought this, but actually that.Tutor Registration →
Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.