Lesson 3: 1.3 Elemental Composition of Pure Substances
Duration of Days: 1
Lesson Objective
Explain the quantitative relationship between the elemental composition by mass and the empirical formula of a pure substance.
How does the Law of Definite Proportions help verify the purity of a chemical sample?
What is the difference between an empirical formula and a molecular formula?
Pure Substance
Law of Definite Proportions
Empirical Formula
Percent Composition
Learning Objective 1.3.A; Suggested Skill 2.A (Question and Method)
This lesson highlights that pure substances contain elements in fixed, definite mass proportions. Students will learn to translate percent composition by mass data into the lowest whole-number ratio of atoms (the empirical formula).
Support: Break empirical formula calculations into a rhythmic 4-step mnemonic: "Percent to mass, mass to mole, divide by small, multiply 'til whole."
Extension: Provide students with a mock lab report containing systematic errors in a percent composition experiment and have them critique the method and predict how the calculated ratio would be affected.
Free-Response Question (FRQ) tasking students to calculate the empirical formula of a compound given its percent composition data, followed by calculating the molecular formula using a provided molar mass. This is found on AP Classroom