New
New
Year 10
Edexcel
Higher

# Empirical formula calculations: using moles

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

New
New
Year 10
Edexcel
Higher

# Empirical formula calculations: using moles

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.
4. Know that we are using moles, and molar ratios to calculate formulae

### 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.

• Mole - A mole of something is 6.02 × 10²³ of it. The mass of a mole of a substance is its relative mass expressed in grams.

This topic is 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).

## Starter quiz

### 6 Questions

Q1.
What is the chemical formula for water?
Q2.
How many particles are in one mole of any substance?
23 × 10⁶
6.23 × 10²⁰
3.60 × 10²
Q3.
Calculate the relative formula mass (Mr) of carbon dioxide (CO₂).
Q4.
If 2 moles of hydrogen gas react with 1 mole of oxygen gas, how many moles of water are produced?
Q5.
What is the mass (g) of 2 moles of carbon dioxide (CO₂)?
Correct Answer: 88 g, 88, 88 grams
Q6.
If a compound is made of 30 g of carbon and 70 g of oxygen, what is the percentage by mass of carbon?

## Exit quiz

### 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 and molecular formula for a compound with a molecular formula of C₆H₁₂O₆.
Q3.
The empirical formula of a compound is NH₃ and its relative formula mass (Mr) is 34. What is its molecular formula?
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.