What is the difference between an empirical formula and a molecular formula?

An empirical formula gives the simplest whole-number ratio of atoms of each element in a compound. A molecular formula gives the actual number of atoms of each element in one molecule.

How are the empirical and molecular formulae of a compound related?

The molecular formula is a whole-number multiple of the empirical formula. The multiple is the relative molecular mass divided by the empirical formula mass.

A sample of a hydrocarbon contains 2.4 g of carbon and 0.6 g of hydrogen, and its relative molecular mass is 30. What is its molecular formula?

The molecular formula is C₂H₆. Carbon gives 2.4 ÷ 12 = 0.2 mol and hydrogen 0.6 ÷ 1 = 0.6 mol, a 1 : 3 ratio. The empirical formula CH₃ has mass 15, and 30 ÷ 15 = 2.

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Organic Chemistry

Organic formulae, homologous series and functional groups

3 things to know
A2 2

Overview

Hydrocarbons, empirical and molecular formulae with worked calculations, structural, skeletal and general formulae, homologous series and functional groups.

What you need to know

Answer each one before you reveal it.

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Key term

A hydrocarbon contains 85.7% carbon by mass and has a relative molecular mass of 56. What are its empirical and molecular formulae?

Key term

A compound contains 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass, and its relative molecular mass is 180. What is its molecular formula?

Key term

What is a hydrocarbon?

Questions from this subtopic

Each one with the model answer underneath.

1
What is a hydrocarbon?
Model answer
A hydrocarbon is a compound made of carbon and hydrogen atoms only. Methane, CH₄, and ethene, C₂H₄, are both hydrocarbons.
2
What is the difference between an empirical formula and a molecular formula?
Model answer
An empirical formula gives the simplest whole-number ratio of atoms of each element in a compound. A molecular formula gives the actual number of atoms of each element in one molecule.
3
How are the empirical and molecular formulae of a compound related?
Model answer
The molecular formula is a whole-number multiple of the empirical formula. The multiple is the relative molecular mass divided by the empirical formula mass.
4
A hydrocarbon contains 85.7% carbon by mass and has a relative molecular mass of 56. What are its empirical and molecular formulae?
Model answer
The empirical formula is CH₂ and the molecular formula is C₄H₈. Hydrogen is 14.3%, and 85.7 ÷ 12 : 14.3 ÷ 1 = 7.14 : 14.3 = 1 : 2. The empirical mass is 14, and 56 ÷ 14 = 4.
5
A compound contains 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass, and its relative molecular mass is 180. What is its molecular formula?
Model answer
The molecular formula is C₆H₁₂O₆. Carbon gives 40.0 ÷ 12 = 3.33 mol and oxygen 53.3 ÷ 16 = 3.33 mol, against 6.7 mol of hydrogen. The 1 : 2 : 1 ratio gives CH₂O, of mass 30, and 180 ÷ 30 = 6.
6
A sample of a hydrocarbon contains 2.4 g of carbon and 0.6 g of hydrogen, and its relative molecular mass is 30. What is its molecular formula?
Model answer
The molecular formula is C₂H₆. Carbon gives 2.4 ÷ 12 = 0.2 mol and hydrogen 0.6 ÷ 1 = 0.6 mol, a 1 : 3 ratio. The empirical formula CH₃ has mass 15, and 30 ÷ 15 = 2.

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