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Empirical and molecular formula: Class 11 practice

The empirical formula is the simplest whole-number ratio of atoms. Read the method, follow the three worked examples, then try the six questions.

What you should know first

  • Mole concept

The idea behind empirical and molecular formula

The empirical formula is the simplest whole-number ratio of atoms. From mass percentages, divide each by atomic mass, divide by the smallest, and round. Molecular formula = n × empirical, where n = molar mass ÷ empirical mass.

Step-by-step method

  1. Assume 100 g; % → grams.
  2. Divide by atomic masses to get moles.
  3. Divide by the smallest and round to whole numbers.
  4. n = M ÷ empirical mass.

Worked example 1 (Basic)

A compound contains 40% C, 6.67% H, 53.33% O and has molar mass 180 g/mol. Find its empirical and molecular formulae.

  1. Moles: C 3.333, H 6.667, O 3.333
  2. Ratio: C 1 : H 2 : O 1 → CH₂O
  3. n = 180/30 = 6
  4. Answer: Empirical CH₂O; molecular = (CH₂O)×6

Worked example 2 (Standard)

A compound contains 80% C, 20% H and has molar mass 30 g/mol. Find its empirical and molecular formulae.

  1. Moles: C 6.667, H 20
  2. Ratio: C 1 : H 3 → CH₃
  3. n = 30/15 = 2
  4. Answer: Empirical CH₃; molecular = (CH₃)×2

Worked example 3 (Challenge)

A compound contains 5.88% H, 94.12% O and has molar mass 34 g/mol. Find its empirical and molecular formulae.

  1. Moles: H 5.882, O 5.882
  2. Ratio: H 1 : O 1 → HO
  3. n = 34/17 = 2
  4. Answer: Empirical HO; molecular = (HO)×2

Common mistakes and misconceptions

  • Rounding 1.5 to 2 instead of doubling
  • Skipping the divide-by-smallest step

Practice questions

  1. (Basic) A compound contains 30.43% N, 69.57% O and has molar mass 46 g/mol. Find its empirical and molecular formulae.
  2. (Basic) A compound contains 30.43% N, 69.57% O and has molar mass 92 g/mol. Find its empirical and molecular formulae.
  3. (Standard) A compound contains 92.31% C, 7.69% H and has molar mass 78 g/mol. Find its empirical and molecular formulae.
  4. (Standard) A compound contains 82.76% C, 17.24% H and has molar mass 58 g/mol. Find its empirical and molecular formulae.
  5. (Challenge) A compound contains 92.31% C, 7.69% H and has molar mass 26 g/mol. Find its empirical and molecular formulae.
  6. (Challenge) A compound contains 85.71% C, 14.29% H and has molar mass 84 g/mol. Find its empirical and molecular formulae.

Answer key with full working

  • Q1: Moles: N 2.174, O 4.348 → Ratio: N 1 : O 2 → NO₂ → n = 46/46 = 1 → Answer: Empirical NO₂; molecular = (NO₂)×1
  • Q2: Moles: N 2.174, O 4.348 → Ratio: N 1 : O 2 → NO₂ → n = 92/46 = 2 → Answer: Empirical NO₂; molecular = (NO₂)×2
  • Q3: Moles: C 7.692, H 7.692 → Ratio: C 1 : H 1 → CH → n = 78/13 = 6 → Answer: Empirical CH; molecular = (CH)×6
  • Q4: Moles: C 6.897, H 17.241 → Ratio: C 2 : H 5 → C₂H₅ → n = 58/29 = 2 → Answer: Empirical C₂H₅; molecular = (C₂H₅)×2
  • Q5: Moles: C 7.692, H 7.692 → Ratio: C 1 : H 1 → CH → n = 26/13 = 2 → Answer: Empirical CH; molecular = (CH)×2
  • Q6: Moles: C 7.143, H 14.286 → Ratio: C 1 : H 2 → CH₂ → n = 84/14 = 6 → Answer: Empirical CH₂; molecular = (CH₂)×6

Syllabus note

This skill is listed under "Some Basic Concepts of Chemistry" for Class 11 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.

Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.

Related practice and notes

Frequently asked questions

Which chapter is empirical and molecular formula in?

"Some Basic Concepts of Chemistry", Class 11 Chemistry (NCERT/CBSE). Other boards may place it differently.

How were the answers checked?

Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.

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