Identifying an organic unknown from data

Step 1: the molecular formula

1. Divide each percentage by the relative atomic mass to get moles.

2. Divide by the smallest to get the simplest ratio: this is the empirical formula.

3. The molecular ion peak (largest m/z, apart from a small M+1 peak) gives Mr. Molecular formula = empirical formula x (Mr / empirical mass).

Step 2: groups from tests and IR

• Use chemical tests to find the functional groups (see the previous lesson).
• IR (wavenumber ranges from the Data Booklet): a broad O-H absorption around 2500 to 3300 cm^-1 shows a carboxylic acid; a broad O-H at higher wavenumber (about 3200 to 3550 cm^-1) an alcohol or phenol; a strong C=O around 1700 cm^-1 a carbonyl group. Explain how you used the table: match the peak's wavenumber to the bond, then the bond to the group.

Step 3: structure from NMR and fragments

• ^13C NMR: number of peaks = number of carbon environments.
• ^1H NMR: number of peaks = proton environments; relative areas = numbers of H; splitting (n + 1 rule) shows neighbouring H.
• Mass spectrum fragments: write each fragment with a + charge, and check its mass matches the m/z value (June 2025 report).
• Draw every structure that fits if the data do not decide between isomers, and say which test would.

Worked example

A white solid contains C 60.87%, H 4.35%, O 34.78%, and its molecular ion peak is at m/z = 138. Find its molecular formula.

1. C: 60.87 / 12.0 = 5.07; H: 4.35 / 1.0 = 4.35; O: 34.78 / 16.0 = 2.17
2. Divide by 2.17: C 2.33, H 2.00, O 1.00
3. Multiply by 3 to get whole numbers: C7H6O3 (empirical mass 138)
4. Mr = 138, so the molecular formula is the same.

Answer: C7H6O3

Check yourself

1. A compound contains C 54.5%, H 9.1% and O 36.4% by mass, and its molecular ion peak is at m/z = 88. How many carbon atoms are in one molecule?
   Show answer

   4. Ratio C : H : O = 2.00 : 4.00 : 1.00, so the empirical formula is C2H4O (mass 44). 88 / 44 = 2, so C4H8O2: 4 carbon atoms.

2. For the same compound (C 54.5%, H 9.1%, O 36.4%, M = 88), how many hydrogen atoms are in one molecule?
   Show answer

   8. C2H4O x 2 = C4H8O2, so 8 hydrogen atoms.

3. An IR spectrum shows a very broad absorption from about 2500 to 3300 cm^-1 and a strong peak near 1710 cm^-1. Which group is present?
   • Alcohol
   • Ketone
   • Carboxylic acid
   • Alkene
   Show answer

   Carboxylic acid. Broad acid O-H plus C=O.

4. What does the m/z value of the molecular ion peak give?
   • The number of carbon atoms
   • The relative molecular mass of the compound
   • The empirical formula
   • The functional group
   Show answer

   The relative molecular mass of the compound. M+ is the whole molecule with one electron removed.

5. Which is a correct formula for the fragment that gives a peak at m/z = 29 in the mass spectrum of propanal?
   • C2H5
   • CH3CH2-
   • C2H5²+
   • C2H5+
   Show answer

   C2H5+. Fragments detected are positive ions; C2H5 has mass 29.

6. How many peaks are in the ^1H NMR spectrum of propanone, CH3COCH3?
   • 2
   • 1
   • 3
   • 6
   Show answer

   1. Both CH3 groups are in the same environment.

7. Compound C3H6O gives an orange precipitate with 2,4-DNPH and a silver mirror with Tollens' reagent. What is it?
   • Propanone
   • Propan-1-ol
   • Prop-2-en-1-ol
   • Propanal
   Show answer

   Propanal. C=O that is an aldehyde, with three carbons.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel IAL Chemistry (2018) specification
• Pearson IAL Chemistry teacher practical guide
• Pearson examiners' report, WCH13 June 2025
• Pearson WCH16 January 2022 question paper (unused)

Last updated 8 November 2026. Spotted a mistake? Tell us.