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).

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6 Questions

Q1.
What is the chemical formula for water?
Correct Answer: H₂O, H2O
Q2.
How many particles are in one mole of any substance?
Correct answer: 6.02 × 10²³
23 × 10⁶
6.23 × 10²⁰
3.60 × 10²
Q3.
Calculate the relative formula mass (Mr) of carbon dioxide (CO₂).
Correct Answer: 44, forty four
Q4.
If 2 moles of hydrogen gas react with 1 mole of oxygen gas, how many moles of water are produced?
Correct Answer: 2, two
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?
Correct Answer: 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 and molecular 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 the following information, calculate the empirical formula of a compound: 2.4 moles of magnesium and 4.8 moles of oxygen.
MgO
Correct answer: MgO₂
Mg₂O₄
Mg₂O
Q6.
Given 40 g of carbon reacts with 10 g of hydrogen to form a compound, calculate the empirical formula of the compound. Use the relative atomic masses (Ar): Carbon = 12, Hydrogen = 1.
CH
Correct answer: CH₄
C₄H
C₄H₄