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S6 Chemistry: Instrumental Analysis (Paper 1: Basics)
S6 Chemistry: Instrumental Analysis (Paper 1: Basics)
Ten items on infrared bands, the molecular ion peak, colorimetric and atomic absorption calculations, with answers.
Direct answers Answers in order: a carboxylic acid, C6H14, 0.25 mol dm-3, 40 ppm, acid-base titration, a carbonyl group, the CH3 positive ion, 0.50, comparing retention times, no.
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)
- FoundationAn infrared spectrum shows a broad band at 3200-3600 cm-1 and a strong band at 1700 cm-1. Which class of compound is most likely?
- FoundationThe molecular ion peak of a hydrocarbon appears at m/z 86. Which formula fits?
- FoundationIn a colorimetric assay a 0.10 mol dm-3 standard gives absorbance 0.20 and the unknown gives 0.50. What is the unknown concentration?
- IntermediateA 5.0 cm3 water sample is diluted to 100 cm3 and gives 2.0 ppm by atomic absorption. What was the original concentration?
- IntermediateWhich of the following is not an instrumental method?
- IntermediateWhat does a strong infrared absorption at 1700 cm-1 indicate?
- ChallengeWhich fragment most likely gives the peak at m/z 15 in a mass spectrum?
- ChallengeA thin layer chromatogram shows a spot 3.0 cm from the origin and a solvent front 6.0 cm from the origin. What is the Rf value?
- ChallengeWhat is the most reliable basis for identifying an unknown by gas chromatography?
- Error redoMisconception drill: can an infrared spectrum give the relative molecular mass directly?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | A carboxylic acid, with both a hydroxyl and a carbonyl group | The broad band comes from a hydrogen bonded OH and the strong 1700 band from C=O, so both groups are present. |
| 2 | C6H14 | The molecular ion sits at the relative molecular mass, and 12 times 6 plus 14 gives 86. |
| 3 | 0.25 mol dm-3 | Absorbance is proportional to concentration, so 0.50 divided by 0.20 gives 2.5 times the standard. |
| 4 | 40 ppm | The dilution factor is 100 divided by 5.0, that is 20, so the measured value is multiplied by 20. |
| 5 | Acid-base titration | Titration relies on an indicator colour change and glassware readings, so it is classical volumetric analysis. |
| 6 | A carbonyl group, C=O | The carbon-oxygen double bond stretches at about 1700 cm-1. |
| 7 | The CH3 positive ion | Carbon at 12 plus three hydrogens gives 15, the methyl fragment minus one electron. |
| 8 | 0.50 | Rf is the spot distance divided by the solvent front distance, so 3.0 over 6.0. |
| 9 | Comparing retention time with an authentic standard | On the same column with the same temperature programme a compound has a fixed retention time. |
| 10 | No, that comes from the molecular ion peak in the mass spectrum | Infrared shows vibrational absorption of functional groups, while mass information comes from mass spectrometry. |
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.
Write \”infrared for groups, mass spectrometry for mass\” on line one of your notes.Tutor Registration →
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