A food scientist is analysing an artificial sweetener to confirm its identity and purity. The sweetener is known to contain only carbon, hydrogen, and oxygen. A sample of the sweetener is analysed and found to contain 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. The relative atomic masses are: C = 12, H = 1, O = 16. The scientist calculates the empirical formula and finds it matches glucose. Explain what is meant by the term empirical formula and describe how the scientist could use melting point data to confirm whether the sample is a pure substance.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A food scientist analyses an artificial sweetener sample. The sample contains only carbon, hydrogen, and oxygen. After analysis, the percentage by mass composition is found to be: carbon 40.0%, hydrogen 6.7%, oxygen 53.3%. The relative atomic masses are C = 12, H = 1, O = 16. Dividing each percentage by its relative atomic mass gives the ratio C : H : O = 3.33 : 6.7 : 3.33, which simplifies to 1 : 2 : 1, giving the empirical formula CH₂O.
Model answer (5 marks)
An empirical formula is the simplest whole‑number ratio of the atoms of each element present in a compound.
For the sample the mass percentages give the ratio C:H:O = 3.33:6.7:3.33, which reduces to 1:2:1, so the empirical formula is CH₂O.
A pure substance has a sharp, specific melting point – it melts at one fixed temperature. A mixture, however, melts over a range of temperatures because its components melt at different temperatures.
Therefore the scientist should determine the melting point of the sample and compare it with the known melting point of pure glucose. If the measured melting point is sharp and matches the accepted value, the sample can be considered pure.
For the sample the mass percentages give the ratio C:H:O = 3.33:6.7:3.33, which reduces to 1:2:1, so the empirical formula is CH₂O.
A pure substance has a sharp, specific melting point – it melts at one fixed temperature. A mixture, however, melts over a range of temperatures because its components melt at different temperatures.
Therefore the scientist should determine the melting point of the sample and compare it with the known melting point of pure glucose. If the measured melting point is sharp and matches the accepted value, the sample can be considered pure.
Examiner tips
- Define empirical formula as simplest whole‑number ratio; give the calculated ratio and formula; explain melting point behaviour of pure vs. mixed substances; state that a sharp, matching melting point confirms purity.
Common mistakes
- Confusing empirical formula with molecular formula; giving the wrong ratio; not mentioning that a pure substance melts at a single temperature; ignoring the need to compare to the known melting point of glucose.
Mark scheme (5 marks)
- The empirical formula is the simplest whole number ratio of atoms of each element in a compound
- The empirical formula for this sample is CH₂O
- A pure substance has a sharp / specific melting point (melts at one fixed temperature)
- A mixture melts over a range of temperatures (not at a single fixed point)
- The scientist should measure / record the melting point of the sample and compare it to the known melting point of pure glucose; if it matches the accepted value (and is sharp), the sample is pure
Key terms in this question
empirical formula · melting point data · pure substance
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
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