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Year 10
Edexcel
Foundation

Empirical formula calculations

I can determine the empirical formulae of simple compounds using reactant masses or percentage composition.

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New
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Year 10
Edexcel
Foundation

Empirical formula calculations

I can determine the empirical formulae of simple compounds using reactant masses or percentage composition.

Download all resources
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Share activities with pupils
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Lesson details

Key learning points

  1. Empirical formula is the simplest whole number ratio of atoms of each element in a compound.
  2. Using the masses of elements reacted together, or their percentage compositions, calculate the empirical formula.
  3. Knowing the relative formula mass of a substance, and its empirical formula, calculate the molecular formula.

Keywords

  • Empirical formula - A substance's empirical formula shows the simplest whole number ratio of atoms of each element in a compound.

  • Relative formula mass - The relative formula mass of a substance is the sum of the relative atomic masses of all the atoms in a formula.

  • Molecular formula - A substance's molecular formula shows the actual number of atoms of each element in a molecule of a compound.

Common misconception

Pupils easily confuse empirical and molecular formula.

Provide multiple examples of formula and challenge pupils to distinguish them as molecular or empirical formula; if the former, push pupils to suggest the molecule's empirical formula.

This lesson provides a useful link to organic chemistry and developing generic formula for different molecules of the same homologous series.
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Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

Lesson video

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Starter quiz

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6 Questions

Q1.
Which of the following is the correct formula for carbon dioxide?
CO
Correct answer: CO₂
C₂O
C₂O₂
2CO
Q2.
Calculate the relative molecular mass (Mr) of carbon dioxide (CO₂).
Correct Answer: 44
Q3.
If two molecules of hydrogen gas react with one molecule of oxygen gas, how many molecules of water are produced?
one
Correct answer: two
three
four
Q4.
A reaction requires three molecules of nitrogen gas and nine molecules of hydrogen gas. What is the simplest ratio of nitrogen to hydrogen needed for this reaction?
3:9
Correct answer: 1:3
9:3
6:2
Q5.
If one unit of carbon dioxide has a mass of 44 grams, what would be the mass of two units of carbon dioxide?
Correct Answer: 88 g, 88, 88 grams
Q6.
If a compound is made of 30g of carbon and 70g of oxygen, what is the percentage by mass of carbon?
Correct Answer: 30%, 30, 30 %

Exit quiz

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6 Questions

Q1.
Which of the following best describes the term "empirical formula"?
The formula that shows the total number of atoms in a molecule.
Correct answer: The simplest whole number ratio of atoms of each element in a compound.
The formula that describes the types of chemical bonds in a compound.
The formula used exclusively in organic chemistry.
Q2.
Write the empirical formula for a compound with a molecular formula of C₆H₁₂O₆.
Correct Answer: CH₂O, CH2O
Q3.
The empirical formula of a compound is NH₃ and its relative formula mass (Mr) is 34. What is its molecular formula?
Correct answer: N₂H₆
NH₃
N₂H₄
NH₂
Q4.
True or false? Empirical formula shows the simplest ratio of how many atoms there are of each element in a substance. Molecular formula shows the exact numbers of atoms of each element.
Correct answer: true
false
Q5.
Given 120g of carbon reacts with 10g of hydrogen to form a compound, calculate the simplest ratio of carbon:hydrogen in the compound. Use the relative atomic masses (Ar): carbon = 12, hydrogen = 1.
Correct answer: 1:1
1:4
4:1
4:4
Q6.
A compound is 85.7% carbon and 14.3% hydrogen by mass. Calculate its empirical formula. Use the relative atomic masses (Ar): carbon = 12, hydrogen = 1.
CH
C₂H₃
Correct answer: CH₂
C₃H₄