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Unit 3: Quantities in Chemical ReactionsActivity 3: Empirical and Molecular FormulasReviewChemical formulas are very important; they:state which elements are in the moleculegive the exact number of atoms of each element that are in the moleculemay give an indication as to how the elements are bonded togetherThe law of definite proportionsIn 1779, Joseph Proust showed that the proportion of the mass of the elements incopper carbonate always stays the same. This is called the law of definite proportions; itis also sometimes called the law of constant composition.When elements combine to form compounds, they do so in a definite proportion.The percentage composition of an element can be expressed as:Water has the chemical formula H2O. In terms of mass, itsmolecule is always made up of 11% hydrogen and thus theremaining mass (89%) must be oxygen. Notice how muchmore massive the oxygen atom is in this image of the watermolecule.Similarly, CO2is always made up of 27.3% carbon.Percentage composition of compoundsThe percentage composition is the percentage of an element in a compound, in relationto its total mass.Example 1Calculate the percentage composition (by mass) of Al2O3.
MM(Al2O3) = 2(27 g/mol) + 3(16 g/mol) = 102 g/mol% Al = 2(27 g/mol)/102 g/mol x 100% = 52.9%

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