Lesson 7: Calculating Chemical Composition

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We are taking a side trip back into the realm of math as we learn how to figure out the numbers of atoms and molecules in certain masses of substances, calculate mass percent composition, and delve into empirical and molecular formulas. Get out those calculators!

Textbook Reading:  Chapter 6, pp. 165-194 (Yes, that includes the examples in the Chapter in Review.)


The measurement techniques in this chapter have many practical applications. For example, chemists and chemical engineers are interested in how much starting material is needed to produce a specific amount of end product most efficiently. As pointed out in the first part of the chapter, health care workers often use these measurements to help ensure patients receive proper care and nutrition. Knowing how to perform these calculations can even help you save money at the gas pump.

food label

Matter can be measured in several ways. You might count how many of something you have, or you might determine the mass or volume. 

The mole, also called Avogadro’s number, is a very large number (6.022 x 1023) that allows us to convert between mass and number of molecules. Moles are used extensively in chemistry and it is very important to understand how they work.

Molar mass is the mass of one mole of any chemical compound, typically with units of g/mol. The lightest possible chemical is hydrogen gas, but there is no limit to how heavy a chemical compound can be. Macromolecules (large organic compounds such as DNA) can weigh thousands of grams per mole.

Mr. Isaacs at IsaacsTeach has 3 short videos that really help to explain moles and how to use them. Pull out your calculator and periodic table (one that lists atomic mass) to work along with the videos.

1. The Mole Concept

2. Molar Mass

3. Molar Volume

Mr. Causey has a video about How to Calculate Percent Composition, Empirical Formulas, etc.


So, you made it through the chapter. How did it go? Be sure to let me know if you have any questions or comments.

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