Explain why the empirical formula mass and the molar mass of a compound are not always equal, and outline how a chemist could use the molar mass of a compound, determined experimentally, to find its molecular formula from its empirical formula.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The empirical formula gives only the simplest whole‑number ratio of atoms in a compound, so its mass is the mass of one mole of that simplest formula.
The actual compound may contain several of these empirical units; its molar mass is therefore a whole‑number multiple (n) of the empirical formula mass.
A chemist finds n by dividing the experimentally determined molar mass by the empirical formula mass.
Multiplying every subscript in the empirical formula by n gives the molecular formula.
The actual compound may contain several of these empirical units; its molar mass is therefore a whole‑number multiple (n) of the empirical formula mass.
A chemist finds n by dividing the experimentally determined molar mass by the empirical formula mass.
Multiplying every subscript in the empirical formula by n gives the molecular formula.
Examiner tips
- Use the word ‘whole‑number multiple’ to show understanding of n.
- Show the calculation n = M_experimental ÷ M_empirical.
- Explain that the empirical formula mass is the mass of one mole of the empirical units.
- State that the molecular formula is obtained by multiplying all subscripts by n.
Common mistakes
- Confusing empirical formula mass with molar mass of the compound.
- Using a fractional n instead of a whole number.
- Failing to show the division step to find n.
Mark scheme (4 marks)
- The empirical formula gives only the simplest whole-number ratio of atoms in a compound, not necessarily the actual number of atoms per molecule.
- The molar mass of the molecular compound is a whole-number multiple (n) of the empirical formula mass.
- The value of n is calculated by dividing the experimentally determined molar mass by the empirical formula mass.
- The molecular formula is then obtained by multiplying all subscripts in the empirical formula by n.
Key terms in this question
empirical formula · molecular formula · molar mass · empirical formula mass
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