http://www.ck12.org Chapter 10. The Mole
10.3 Chemical Formulas
Lesson Objectives
- Calculate the percent composition of a compound either from mass data or from the chemical formula. Use
percent composition to calculate the mass of an element in a certain sample of a compound. - Calculate the percentage of a hydrate’s mass that is due to water.
- Determine the empirical formula of a compound from percent composition data.
- Determine the molecular formula of a compound from the empirical formula and the molar mass.
Lesson Vocabulary
- hydrate
- percent composition
Check Your Understanding
Recalling Prior Knowledge
- How is the mass of an element or compound converted to moles?
- What is an empirical formula? What is a molecular formula, and how does it relate to an empirical formula?
In previous chapters, you have learned about chemical nomenclature –naming compounds and writing correct
chemical formulas. In this lesson, you will learn how the subscripts in a chemical formula represent the molar
ratio between the elements in a compound.
Percent Composition
Packaged foods that you eat typically have nutritional information provided on the label. The label of a popular
brand of peanut butter (Figure10.12) reveals that one serving size is considered to be 32 g. The label also gives the
masses of various types of compounds that are present in each serving. One serving contains 7 g of protein, 15 g of
fat, and 3 g of sugar. This information can be used to determine the composition of the peanut butter on a percent
by mass basis. For example, to calculate the percent of protein in the peanut butter, we could perform the following
calculation:
7 g protein
32 g×100%=22% protein