New
New
Year 10
Edexcel
Foundation

Empirical formula calculations

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

New
New
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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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.

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.

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.

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

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).

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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 %

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