Explain how the molecular formula of a compound can be determined from its empirical formula and its molar mass, using the example of a compound with the empirical formula CH₂O and a molar mass of 180 g mol⁻¹.

IB DP Chemistry Standard Level (2023 syllabus) — S1.4 Counting particles by mass: The mole · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The empirical formula mass of CH₂O is 30 g mol⁻¹ (12+2×1+16). Dividing the molar mass of the compound (180 g mol⁻¹) by this gives 180 ÷ 30 = 6, so the empirical formula must be repeated six times. Multiplying each subscript in CH₂O by 6 gives C₆H₁₂O₆, which is the molecular formula.

Examiner tips

  • Show the calculation of the empirical formula mass first; examiners look for the 30 g mol⁻¹ figure.
  • Explain the division step and state the whole‑number ratio n=6.
  • State that the empirical formula is multiplied by n to give the molecular formula.
  • Write the final formula clearly as C₆H₁₂O₆.

Common mistakes

  • Using the wrong empirical formula mass (e.g., 28 g mol⁻¹)
  • Failing to show the division step or giving a non‑whole number ratio
  • Not multiplying all subscripts by the same factor

Mark scheme (4 marks)

  1. Calculate the empirical formula mass of CH₂O as 30 g mol⁻¹
  2. Divide the molar mass by the empirical formula mass to obtain the whole-number ratio (n = 180 ÷ 30 = 6)
  3. Multiply all subscripts in the empirical formula by n to give the molecular formula
  4. The molecular formula is C₆H₁₂O₆

Key terms in this question

empirical formula · molecular formula · molar mass

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