Relative Formula Mass (Mr)

Active learning works well for relative formula mass because students often confuse the mole as a unit of mass or volume. By handling real or simulated masses and counting particles, they build the critical distinction between amount, mass, and volume.

• Year 10
• Chemistry
• 3 activities
• 20 min–35 min

Learning Objectives

1. Calculate the relative formula mass (Mr) for given ionic and covalent compounds using provided relative atomic masses.
2. Explain the relationship between relative atomic mass (Ar) and relative formula mass (Mr) in chemical calculations.
3. Identify the constituent elements and their quantities within a chemical formula to determine Mr.
4. Analyze how a change in the relative atomic mass of an element would affect the calculated Mr of a compound.

Inquiry Circle

⏱ 35 min·Small Groups

Inquiry Circle: Counting by Weighing

Give students bags of different items (e.g., rice, pasta, beans). They must find the mass of 10 items, then use that to 'count' how many items are in a mystery bag by weighing it, mimicking how chemists use the mole.

Prepare & details

Calculate the relative formula mass for various ionic and covalent compounds.

Facilitation Tip: During Collaborative Investigation: Counting by Weighing, circulate and ask groups to predict the mass of 0.5 mol of each sample before they weigh it.

Setup: Groups at tables with access to source materials

Materials: Source material collection, Inquiry cycle worksheet, Question generation protocol, Findings presentation template

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Stations Rotation

⏱ 25 min·Pairs

Stations Rotation: Mr Calculations

Set up stations with different chemical formulas and a periodic table. Students move through stations to calculate the relative formula mass, checking their answers against a hidden key to gain 'points' for their team.

Prepare & details

Explain the significance of relative formula mass in quantitative chemistry.

Facilitation Tip: During Station Rotation: Mr Calculations, ensure each station includes a worked example card for students to reference as they move between tasks.

Setup: Tables/desks arranged in 4-6 distinct stations around room

Materials: Station instruction cards, Different materials per station, Rotation timer

• Remember
• Understand
• Apply
• Analyze
• Self-Management
• Relationship Skills

Think-Pair-Share

⏱ 20 min·Pairs

Think-Pair-Share: The Mole Map

Students are given a partially completed 'mole map' showing the relationships between mass, moles, and Mr. They work in pairs to fill in the missing arrows and formulas, then explain the map to another pair.

Prepare & details

Analyze how errors in determining relative atomic masses would impact formula mass calculations.

Facilitation Tip: During Think-Pair-Share: The Mole Map, assign roles (recorder, reporter) within pairs to keep all students engaged in the discussion.

Setup: Standard classroom seating; students turn to a neighbor

Materials: Discussion prompt (projected or printed), Optional: recording sheet for pairs

• Understand
• Apply
• Analyze
• Self-Awareness
• Relationship Skills

Teaching This Topic

Teach this topic by first anchoring the mole to a familiar counting unit like dozens or pairs. Use analogies that resonate with students’ everyday experiences before moving to abstract calculations. Avoid rushing to formulas; instead, build understanding through concrete examples and repeated practice with feedback.

What to Expect

Successful learning looks like students confidently distinguishing between relative atomic mass and relative formula mass and correctly calculating Mr for unfamiliar compounds. They should also explain why the mole is a counting unit, not a mass or volume unit, using their own words and examples.

Watch Out for These Misconceptions

Common Misconception

During Collaborative Investigation: Counting by Weighing, watch for students assuming that equal masses of different substances contain the same number of particles.

What to Teach Instead

Use the investigation to explicitly compare equal masses of different substances and ask students to count the number of particles in each sample using Avogadro’s constant, highlighting the disparity.

Common Misconception

During Station Rotation: Mr Calculations, watch for students treating the mole as a unit of volume.

What to Teach Instead

Include a sorting task at one station where students categorize statements about mass, volume, and moles, reinforcing that the mole is a measure of amount, not volume.

Assessment Ideas

Quick Check

After Station Rotation: Mr Calculations, provide a list of chemical formulas (e.g., CO2, NaCl, H2SO4, Mg(OH)2) and ask students to calculate the Mr for each compound and show their working. Collect these to identify common errors in addition or multiplication.

Exit Ticket

After Collaborative Investigation: Counting by Weighing, ask students to write down the chemical formula for sulfuric acid and then calculate its relative formula mass. Also, ask them to explain in one sentence why knowing the Mr is important for chemists.

Discussion Prompt

During Think-Pair-Share: The Mole Map, pose the question: 'If the accepted relative atomic mass for oxygen changed from 16.0 to 16.1, how would this affect the calculated relative formula mass of carbon dioxide (CO2)?' Facilitate a class discussion on how errors in Ar propagate to Mr.

Extensions & Scaffolding

• Challenge early finishers to design a card game where players match chemical formulas with their Mr values.
• Scaffolding for struggling students: provide a step-by-step template for Mr calculations with each step color-coded (e.g., identify elements, find Ar, multiply by subscripts, sum).
• Deeper exploration: ask students to research how relative atomic masses are determined experimentally and present their findings to the class.

Key Vocabulary

Relative Atomic Mass (Ar)	The weighted mean mass of an atom of an element compared to one-twelfth of the mass of an atom of carbon-12. It is a dimensionless quantity.
Relative Formula Mass (Mr)	The sum of the relative atomic masses of all the atoms in a formula unit of an ionic compound or molecule. It is a dimensionless quantity.
Chemical Formula	A representation of a chemical compound that shows the types of atoms present and the ratio of their numbers. For example, H₂O represents water.
Molecule	A group of two or more atoms held together by chemical bonds. The relative formula mass of a covalent compound is often referred to as its relative molecular mass.
Formula Unit	The simplest whole-number ratio of ions in an ionic compound. The relative formula mass of an ionic compound is calculated from this unit.