1. Chemical Formulas

2. Mass Percent Also known as percent composition. Shows the mass of each element in a compound as a percent of the total mass of the compound.

3. Example What is the mass percent of each element in carbon dioxide, CO 2 ? 1.Find the molar mass of the compound. 2.Find the percent of each element in the compound.

4. More Examples 1.Find the mass percent of each element in mercury (I) chloride, Hg 2 Cl 2. 2.Find the mass percent of each element in ethyne, C 2 H 2. 3.Find the mass percent of each element in benzene, C 6 H 6.

5. Why did you get the same answers for the last two examples? The two compounds have different molecular formulas. Molecular formula – the formula showing exactly the number of atoms of each element in one molecule of a compound. The compounds, however, have the same empirical formula. Empirical formula – the formula showing the lowest whole number ratio of atoms of each element in a compound.

6. Empirical formulas In many cases, the molecular formula for a compound is the same as the empirical formula. For example – H 2 O, CH 4, SO 3, C 12 H 22 O 11 However, many molecules contain multiples of atoms that can be reduced to a smaller ratio. For example – C 6 H 12 O 6, C 3 H 6, Hg 2 Cl 2 What are the empirical formulas for these examples? Molecular formula = (empirical formula) n