Free Analytical techniques questions for AS Level Chemistry (9701), part of Organic chemistry. Each question has the answer and the marking points an examiner would look for.
Generate a full Analytical Techniques worksheet →All Organic chemistry questionsIn the mass spectrum of an organic compound, the M peak has relative abundance 60 and the M+1 peak has relative abundance 4.
How many carbon atoms are in one molecule of the compound?
Answer: C
Which infrared absorptions are shown by ethyl ethanoate?
Answer: D
Which statement describes infrared spectroscopy?
Answer: C
Infrared spectroscopy and mass spectrometry identify organic compounds.
State the absorption ranges that identify O–H in an alcohol and C=O.
A mass spectrum shows M and M+1 peaks in the ratio 100 : 3.3. Deduce the number of carbon atoms.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 3200–3600; 1680–1750 O–H 3200–3600 cm−1 B1 C=O 1680–1750 cm−1 B1 | 2 |
| (b) | 3 n = (3.3/100) × (100/1.1) C1 3 A1 | 2 |
An organic compound X contains carbon, hydrogen and oxygen only. Its mass spectrum shows a molecular ion peak at m/e = 74 with an M+1 peak of relative height 3.3% of the M peak.
Explain the origin of the M+1 peak.
Use the M+1 peak to deduce the number of carbon atoms in a molecule of X.
The infrared spectrum of X shows a broad absorption at 2500–3000 cm−1 and a strong absorption at 1710 cm−1. Identify X and explain your reasoning.
X reacts with ethanol in the presence of concentrated sulfuric acid. Name the product and state how the IR spectrum of the product differs from that of X.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | molecules containing one carbon-13 atom molecules that contain one ¹³C atom (natural abundance 1.1%) B1 | 1 |
| (b) | 3 n = (M+1 height / M height) × 100/1.1 M1 3 A1 | 2 |
| (c) | propanoic acid broad O–H of a carboxylic acid B1 C=O at 1710 cm−1 B1 propanoic acid, CH₃CH₂COOH B1 | 3 |
| (d) | ethyl ester; no broad O–H; C=O and C–O remain ethyl propanoate B1 the broad O–H absorption disappears B1 C=O (~1740) and C–O (~1200) absorptions remain B1 | 3 |
Compound X contains carbon, hydrogen and oxygen only. It contains 54.5% C and 9.1% H by mass. In its mass spectrum, the molecular ion peak is at m/e = 88 and the M+1 peak has 4.4% of the abundance of the M peak. X shows a strong IR absorption at 1740 cm−1 and no broad absorption above 2500 cm−1.
Calculate the empirical formula of X.
Use the M+1 peak to confirm the number of carbon atoms, and deduce the molecular formula.
Use the infrared data to identify the functional group, and suggest two possible structures for X.
X is hydrolysed by heating with dilute acid. One product is ethanol. Identify X and write the equation for the hydrolysis.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | C2H4O % O = 36.4 C1 mole ratio 4.54 : 9.1 : 2.27 C1 C2H4O A1 | 3 |
| (b) | 4 carbon atoms; C4H8O2 4.4 ÷ 1.1 = 4 B1 C4H8O2 B1 | 2 |
| (c) | ester (C=O at 1740, no O–H); e.g. ethyl ethanoate CH3COOCH2CH3 and methyl propanoate CH3CH2COOCH3 ester B1 first correct structure B1 second correct structure B1 | 3 |
| (d) | ethyl ethanoate; CH3COOC2H5 + H2O ⇌ CH3COOH + C2H5OH ethyl ethanoate B1 balanced equation B1 | 2 |
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