Chemical Formulas

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.

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.

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.

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.

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