Question 1 Medium
A compound contains 1.2 g of carbon and 0.4 g of hydrogen. What is the empirical formula? (Ar: C = 12, H = 1)
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Answer: CH₄
- C: 1.2 ÷ 12 = 0.1 mol. H: 0.4 ÷ 1 = 0.4 mol.
- The ratio is 1 : 4, so CH₄.
Empirical formulae and percentage composition: GCSE Combined Science Chemistry practice questions
A second set of questions on empirical formulae and percentage composition. They are different from the ones on the topic page. Try each, then open the answer and compare your working.
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Question 2 Medium
0.48 g of magnesium burns to make 0.80 g of magnesium oxide. What mass of oxygen joined the magnesium? Give your answer in g.
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Answer: 0.32 g
- Oxygen = mass of oxide − mass of magnesium.
- 0.80 − 0.48 = 0.32 g.
Question 3 Hard
5.6 g of iron combines with 2.4 g of oxygen. What is the empirical formula? (Ar: Fe = 56, O = 16)
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Answer: Fe₂O₃
- Fe: 5.6 ÷ 56 = 0.10. O: 2.4 ÷ 16 = 0.15.
- The ratio 1 : 1.5 becomes 2 : 3.
Question 4 Hard
A hydrocarbon is 85.7% carbon and 14.3% hydrogen by mass. What is the empirical formula? (Ar: C = 12, H = 1)
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Answer: CH₂
- C: 85.7 ÷ 12 = 7.14. H: 14.3 ÷ 1 = 14.3.
- The ratio is 1 : 2.
Question 5 Hard
The empirical formula is CH₂ and the Mr is 42. What is the molecular formula? (Ar: C = 12, H = 1)
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Answer: C₃H₆
- CH₂ has Mr 14.
- 42 ÷ 14 = 3, so C₃H₆.
Question 6 Hard
2.3 g of sodium combines with 0.8 g of oxygen. What is the empirical formula? (Ar: Na = 23, O = 16)
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Answer: Na₂O
- Na: 2.3 ÷ 23 = 0.10. O: 0.8 ÷ 16 = 0.050.
- The ratio is 2 : 1.
Question 7 Hard
The molecular formula of glucose is C₆H₁₂O₆ and its empirical formula is CH₂O. By what number is the empirical formula multiplied?
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Answer: 6
- Compare the number of carbon atoms: 6 and 1.
- The multiple is 6.
Question 8 Hard
Why is the lid of the crucible lifted a little during the magnesium oxide experiment? To let in …
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Answer: oxygen
- Magnesium needs oxygen to burn.
- The lid is lifted to let oxygen in.
Quick checks
Short questions with options. Choose one before you open the answer.
Check 1
What is an empirical formula?
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The simplest whole number ratio of atoms of each element in a compound.
Check 2
What is a molecular formula?
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The actual number of atoms of each element in one molecule.
Check 3
What is the empirical formula of C₂H₆?
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CH₃.
Check 4
What is the empirical formula of H₂O₂?
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HO.
Check 5
What is the empirical formula of glucose, C₆H₁₂O₆?
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CH₂O.
Check 6
Why are ionic compounds always written as an empirical formula?
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They have giant lattices, so only the ratio of ions is known.
Check 7
What is the formula for percentage by mass of an element?
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Ar × number of atoms ÷ Mr × 100.
Check 8
What is the first step to find an empirical formula from masses?
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Divide each mass by the Ar of the element to get moles.
Check 9
What is the second step to find an empirical formula?
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Divide each number of moles by the smallest number of moles.
Check 10
What do you do if the ratio is 1 : 1.5?
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Multiply both by 2 to get 2 : 3.
Check 11
What do you do with percentages?
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Treat them as masses in grams for a 100 g sample.
Check 12
How do you find the molecular formula from the empirical formula?
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Divide Mr by the empirical formula mass and multiply the empirical formula by that number.
Check 13
What is the empirical formula mass of CH₂?
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14.
Check 14
What is the Mr of CH₂O?
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30.
Check 15
In the MgO experiment, what is the mass of oxygen?
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The final mass minus the mass of the magnesium (the mass gain).
Check 16
Why is the lid lifted during the MgO experiment?
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To let oxygen in.
Check 17
Why must smoke not escape in the MgO experiment?
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Magnesium oxide would be lost, so the oxygen mass looks too small.
Check 18
What does the mass of the crucible and contents do in the MgO experiment?
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It increases because oxygen joins the magnesium.
Check 19
What is the empirical formula of magnesium oxide?
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MgO.
Check 20
How many moles are in 0.48 g of magnesium (Ar 24)?
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0.02 mol.
Check 21
What is the percentage of magnesium in MgO?
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60%.
Check 22
Why must you use the Mr given in the question for the molecular formula?
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It tells you the real size of the molecule.
Check 23
What do percentages by mass add up to?
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100%.
Check 24
What is relative formula mass?
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The sum of the relative atomic masses of the atoms in the formula.
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