Mastering Relative Formula Mass: Atoms, Molecules, and Compounds

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Learning Objective

At the end of the lesson, you will be able to calculate the relative formula mass of substances, including molecules and ionic compounds.

What do you already know about calculating relative formula mass?

Understanding Relative Atomic Mass

Relative atomic mass (Ar) compares an element's atoms to carbon-12. Ar values are found in the periodic table and have no units.

Calculating Relative Formula Mass

Relative formula mass (Mr) is the sum of Ar values of atoms in a formula. It applies to molecules and ionic compounds.

Step-by-Step Calculation: CO2

CO2: 1 carbon (Ar = 12) + 2 oxygen (Ar = 16). Mr = 12 + 16 + 16 = 44.

Interactive Activity: Calculation Practice

Calculate Mr for H2O. H (Ar = 1), O (Ar = 16).

Ionic Compounds and Mr

Ionic compounds like NaCl don't form molecules, yet their Mr is calculated similarly.

Worked Example: Calcium Hydroxide

Ca(OH)2: Ca (Ar = 40), O (Ar = 16), H (Ar = 1). Mr = 40 + 2(16 + 1).

Challenge Question: Magnesium Nitrate

Calculate Mr for Mg(NO3)2. Mg (Ar = 24), N (Ar = 14), O (Ar = 16).

Common Mistakes to Avoid

Double-check atom counts and Ar values. Brackets indicate grouped atoms.

Review and Questions

Review key concepts. Answer any questions students may have.

Write down 3 things you learned in this lesson.

Write down 2 things you want to know more about.

Ask 1 question about something you haven't quite understood yet.