Relative atomic and molecular/formula mass

• Chemistry
• Secondary Education

TEMPLATE

Topic 7 - Relative atomic and molecular masses

Masses of Atoms and Molecules

24 November 2025

How do you feel today?

😒🙁😐🙂😃

TEMPLATE

• I will know what relative atomic and molecular masses are.
• I will be able to calculate the relative molecular/formula mass of a compound.

How well did you participate
in the lesson today?

05

TEMPLATE

Demo:

What happens to the mass of a 5-gram magnesium ribbon when it is burnt completely?

What happens to the mass of a 5-gram magnesium ribbon when it is burnt completely?

What happens to the mass of a 5-gram magnesium ribbon when it is burnt completely?

increases

decreases

remains the same

TEMPLATE

Demo:

What happens to the mass of a 5-gram magnesium ribbon when it is burnt completely?

Explain why the mass of a 5-gram magnesium 

ribbon, when it is burnt completely, increases.

TEMPLATE

Work in groups:

You Mass BIN! Tired 🥱

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Relative Atomic Mass (Ar) = the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of ¹²C

TEMPLATE

Relative Atomic Mass (Ar) = the average mass of the isotopes of an element compared to 1/12th of the mass of an atom of ¹²C

Atomic number (Number of protons +)

Relative atomic mass

TEMPLATE

Relative Molecular Mass (Mr) = the sum of the relative atomic masses  of the atoms in a covalent molecule

12 + 16 + 16

Relative Molecular Mass (MrCO2) = 44 

TEMPLATE

Relative Molecular Mass (Mr) = the sum of the relative atomic masses of covalent compounds

1 + 1 + 16

Relative Molecular Mass (MrH2O) = 18

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Relative Formula Mass (Mr) = the sum of the relative atomic masses of the ions in ionic compounds

23 + 35.5

Relative Formula Mass (MrNaCl) = 58.5

TEMPLATE

Relative Formula Mass (Mr) = the sum of the relative atomic masses of ionic compounds

24 + 12 + 16 + 16 + 16

Relative Formula Mass (MrMgCO3) = 84

TEMPLATE

Work in pairs - Page 5 of your LAP Sheets

TEMPLATE

Work independently:

Pages 4-6 of your LAP Sheets

How well did you participate
in the lesson today?

05

TEMPLATE

Assesses student comprehension through a worksheet, quiz, a lab report, or a concluding class discussion.