Percent Composition and Chemical Formulas Prentice-Hall Chapter 10.3 Dr. Yager

Objectives  Describe how to calculate the percent of mass of an element in a compound.  Interpret an empirical formula.  Distinguish between empirical and molecular formulas.

Percent Composition of a Compound The percent by mass of an element in a compound is the number of grams of the element divided by the mass in grams of the compound, multiplied by 100%.

Percent Composition from Mass Data The relative amounts of the elements in a compound are expressed as the percent composition or the percent by mass of each element in the compound.

Percent Composition from the Chemical Formula

Percent Composition as a Conversion Factor You can use percent composition to calculate the number of grams of any element in a specific mass of a compound.

Propane (C 3 H 8 ) is 81.8% carbon and 18% hydrogen. You can calculate the mass of carbon and the mass of hydrogen in an 82.0 g sample of C 3 H 8.

Ethyne (C 2 H 2 ) is a gas used in welder’s torches. Styrene (C 8 H 8 ) is used in making polystyrene. These two compounds of carbon have the same empirical formula (CH) but different molecular formulas.

Empirical Formulas The empirical formula of a compound shows the smallest whole-number ratio of the atoms in a compound.

The molecular formula of a compound is either the same as its experimentally determined empirical formula, or it is a simple whole-number multiple of its empirical formula. Molecular Formula Compared to Empirical Formula

Methonal, ethanoic acid, and glucose all have the same empirical formula - CH 2 O.

1. Calculate the percent by mass of carbon in cadaverine, C 5 H 14 N 2, a compound present in rotting meat. 1. Calculate the percent by mass of carbon in cadaverine, C 5 H 14 N 2, a compound present in rotting meat. a)67.4% C b)58.8% C c)51.7% C d)68.2% C

1. Calculate the percent by mass of carbon in cadaverine, C 5 H 14 N 2, a compound present in rotting meat. 1. Calculate the percent by mass of carbon in cadaverine, C 5 H 14 N 2, a compound present in rotting meat. a)67.4% C b)58.8% C c)51.7% C d)68.2% C

2. Which of the following is NOT an empirical formula? a)NO 2 b)H 2 N c)CH d)C 3 H 6

2. Which of the following is NOT an empirical formula? a)NO 2 b)H 2 N c)CH d)C 3 H 6

3. Determine the molecular formula of a compound that contains 40.0 percent C, 6.71 percent H, and percent O and has a molar mass of g. a)C 2 H 4 O 2 b)CH 2 O c)C 2 H 3 O d)C 2 H 4 O

3. Determine the molecular formula of a compound that contains 40.0 percent C, 6.71 percent H, and percent O and has a molar mass of g. a)C 2 H 4 O 2 b)CH 2 O c)C 2 H 3 O d)C 2 H 4 O