1.1 What is Chemistry?

http://www.ck12.org Chapter 10. The Mole

10.3 Chemical Formulas

Lesson Objectives

• Use a chemical formula or mass data to calculate the percent composition of a compound.


• Use the percent composition of a compound to calculate the mass of an element in a given sample.


• Be able to calculate the empirical formula for a compound when given percent composition data.


• Be able to calculate the molecular formula for a compound when you know its molar mass and its empirical
  formula.

Lesson Vocabulary

• percent composition: The percent by mass of each element in a compound.


• empirical formula: An elemental formula showing the lowest whole-number ratio of the elements in a
  compound.


• molecular formula: A formula showing how many atoms of each element are present in one molecule of a
  molecular compound.

Check Your Understanding

• How can you calculate the amount of a substance in moles from its mass and its molar mass?

Introduction

Packaged foods that you eat typically have nutritional information provided on the label. The label of a popular
brand of peanut butter (Figure10.4) 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

In a similar way, chemists often need to know what elements are present in a compound and in what percentages.
Thepercent compositionis the percent by mass of each element in a compound. It is calculated in a way that is
similar to what we just saw for the peanut butter.