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Percentage composition and empirical formula

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The empirical formula of a compound is the simplest whole number ratio of each type of atom in a compound. It can be the same as the compound’s molecular formula - but not always. An empirical formula can be calculated from information about the mass of each element in a compound or from the percentage composition.

# Empirical formula

## Masses of elements in a compound

The total mass [mass: The amount of matter an object contains. Mass is measured in 'kg'. of a compound [compound: A substance formed by the chemical union (involving bond formation) of two or more elements. is equal to the masses of all the elements [element: A substance made of one type of atom only. it contains. For example, 18 g of water (H2O) contains 2 g of hydrogen atoms [atom: All elements are made of atoms. An atom consists of a nucleus containing protons and neutrons, surrounded by electrons. and 16 g of oxygen atoms.

This principle lets you work out the mass of an element in a compound if you know the mass of the compound, and the masses of the other elements in it.

## Example

100 g of calcium carbonate CaCO3 contains 40 g of calcium and 12 g of carbon. What mass of oxygen does it contain?

mass of oxygen = 100 – 40 – 12 = 48 g

## Empirical formula

The molecular formula of a substance is the actual number of each type of atom in one unit of the substance. For example, the molecular formula of ethane is C2H6 because each ethane molecule contains 2 carbon atoms and 6 hydrogen atoms.

An empirical formula is the simplest whole number ratio of each type of atom in a compound. For example, the empirical formula of ethane is CH3 because both numbers in its molecular formula can be simplified by dividing by 2.

Note that C0.33H (found by dividing by 6) would be incorrect because 0.33 is not a whole number.

The empirical formula and molecular formula can be the same for some compounds. For example, they are the same for carbon dioxide CO2 and methane CH4 because the numbers in their molecular formulae are already in their simplest whole number ratios.

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