The first step in determining percent composition is to determine:

The first step in determining percent composition is to determine:

• A. the empirical formula of the compound
• B. the molar mass of the entire compound
• C. the molar mass of each element in the compound
• D. the percent yield of the reaction

Answer: (C)

Percent composition shows how the mass of a compound is split among its elements. To find these percentages, you first determine the mass contributed by each element in one mole of the compound. Look at the formula, count how many atoms of each element are present, and multiply by that element’s atomic mass to get the mass that element contributes per mole. Add those partial masses to get the total molar mass of the compound, then divide each element’s contributed mass by the total and multiply by 100 to obtain its percent composition. For example, in CO2 there is 1 carbon (12.01 g/mol) and 2 oxygens (2 × 15.999 g/mol = 31.998 g/mol), giving a total of about 44.01 g/mol; carbon contributes 12.01 g, which is about 27.3%, and oxygen contributes about 72.7%. The essential starting step is determining the masses contributed by each element.

Percent composition shows how the mass of a compound is split among its elements. To find these percentages, you first determine the mass contributed by each element in one mole of the compound. Look at the formula, count how many atoms of each element are present, and multiply by that element’s atomic mass to get the mass that element contributes per mole. Add those partial masses to get the total molar mass of the compound, then divide each element’s contributed mass by the total and multiply by 100 to obtain its percent composition. For example, in CO2 there is 1 carbon (12.01 g/mol) and 2 oxygens (2 × 15.999 g/mol = 31.998 g/mol), giving a total of about 44.01 g/mol; carbon contributes 12.01 g, which is about 27.3%, and oxygen contributes about 72.7%. The essential starting step is determining the masses contributed by each element.