Question 1Warm-upHigher
A compound contains 12 g of carbon and 4 g of hydrogen. What is its empirical formula? (C = 12, H = 1)
Moles of C and H, then the ratio.
Worked solution
- C: ; H:
- Ratio 1 : 4, so
Answer:
GCSE & IGCSE chemistry practice
Empirical formula questions and answers
The empirical formula is the simplest whole-number ratio of atoms in a compound. To find it from masses (or percentages): divide each by its , divide by the smallest answer, then round to whole numbers. The molecular formula is a whole-number multiple of it. Relative atomic masses: H = 1, C = 12, N = 14, O = 16, Na = 23, Mg = 24, Al = 27, S = 32, Cl = 35.5, K = 39, Ca = 40, Fe = 56, Cu = 63.5.
8 questions · Higher tier · Every answer is checked as you type it, with a full worked solution. Updated .
Where empirical formula comes up
| Cambridge IGCSE (0620) | Supplement (Extended): define empirical formula and calculate empirical and molecular formulae from data. |
|---|---|
| GCSE (AQA) | AQA GCSE: work out the empirical formula of an ionic compound from a model or diagram of its structure (calculations from masses aren't required). |
The method
- Write the mass (or %) of each element.
- Divide each by its Ar to get moles.
- Divide every result by the smallest.
- If you get .5, double everything. Write the formula.
- Molecular formula: Mr ÷ empirical formula mass gives the multiplier.
Empirical formula questions
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Warm-up
Question 2Warm-upHigher
A compound contains 2.4 g of magnesium and 1.6 g of oxygen. What is its empirical formula? (Mg = 24, O = 16)
Divide each by its Ar.
Worked solution
- Mg: ; O:
- Ratio 1 : 1, so MgO
Answer: MgO
Exam-style
Question 3Exam-styleHigher
A compound is 40% carbon, 6.7% hydrogen and 53.3% oxygen by mass. What is its empirical formula? (C = 12, H = 1, O = 16)
Treat the percentages as grams in 100 g.
Worked solution
- C: ; H: ; O:
- Divide by 3.33: 1 : 2 : 1, so
Answer:
Question 4Exam-styleHigher
5.6 g of iron combines with 2.4 g of oxygen. What is the empirical formula? (Fe = 56, O = 16)
You'll get 1 : 1.5, so double.
Worked solution
- Fe: ; O:
- Ratio 1 : 1.5, doubled to 2 : 3:
Answer:
Question 5Exam-styleHigher
A compound has the empirical formula and . What is its molecular formula?
CH₂ has a mass of 14.
Worked solution
Answer:
Question 6Exam-styleHigher
A compound contains 0.46 g of sodium, 0.32 g of sulfur and 0.64 g of oxygen. What is its empirical formula? (Na = 23, S = 32, O = 16)
Divide by Ar, then by the smallest.
Worked solution
- Na: ; S: ; O:
- Divide by 0.01: 2 : 1 : 4, so
Answer:
Challenge
Question 7ChallengeHigher
A hydrocarbon is 85.7% carbon by mass and has . What is its molecular formula? (C = 12, H = 1)
Find the empirical formula first: hydrogen is 14.3%.
Worked solution
- C: ; H: , so 1 : 2, (mass 14)
- , so
Answer:
Question 8ChallengeHigher
In the ratio step, a student gets C : H = 1 : 2.5. What should they do?
A .5 means double.
Worked solution
- A ratio must be whole numbers; gives 2 : 5.
Answer: Multiply by 2:
Common mistakes
- Dividing by the atomic number instead of Ar.
- Rounding 1.5 to 2 instead of doubling.
- Giving the molecular formula when the empirical formula was asked for.
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Sources
The questions are our own. The “where it comes up” notes are from each board's published specification, checked on 27 October 2026:
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