Relative Formula Mass & Mole Calculations
Relative Formula Mass and Moles
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Relative Formula Mass and Moles
Central question: how do chemists move from a chemical formula to a measurable mass? Keep the opening practical: students often see formulae as symbols only, but this lesson shows how formulae connect to quantities.
Learning goal
I can calculate relative formula mass and use it to work out moles and mass.
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What words do you link with moles and Mr?
Use this to surface prior knowledge and misconceptions such as confusing Mr with grams or seeing moles only as a formula. Pick out any useful vocabulary students already know: atom, mass, formula, periodic table, calculation.
Ar and Mr
Ar means relative atomic mass
It is the mass of one atom compared with carbon-12
Mr means relative formula mass
It is found by adding together the Ar values in a formula
Emphasise that students use Ar values from the periodic table. At this stage, treat Mr for ionic compounds and molecules in the same calculation way: add the relative masses in the formula.
Finding Mr from a formula
To calculate Mr, count each type of atom and add their masses.
H₂O: (2 × 1) + 16 = 18
CO₂: 12 + (2 × 16) = 44
MgCl₂: 24.3 + (2 × 35.5) = 95.3
Model careful reading of subscripts. Students commonly forget to multiply atoms outside brackets or miss the 2 in formulas such as CO2 and MgCl2.
Formulae with more than one part
Add every atom shown in the formula
Multiply atoms inside brackets by the number outside
Keep decimal Ar values when needed
Check that no element has been missed
Ca(OH)₂: 40.1 + 2(16 + 1) = 74.1
If useful, point out that students should write a working line before using a calculator. This reduces errors more effectively than mental calculation alone.
Checks for Mr answers
Larger formulas should give larger Mr values
Missing a subscript gives an answer that is too small
Mr has no unit
A correct formula is essential before any later calculation
This slide exists to build accuracy before moving into moles. Students often write units such as g on Mr; reserve grams for mass, not relative formula mass.
What does Mr mean?
Relative formula mass
Relative atomic mass
Mass in grams
Number of moles
Checks the Ar/Mr distinction and the common error of treating Mr as a measured mass.
Complete the summary
To find Mr, add the Ar values for all atoms in the formula. In Ca(OH)₂, the bracket means the O and H are counted twice. Mr has no unit, so grams should not be written.
Echoes the language from slides 2 to 5: add Ar values, multiply atoms in brackets, and remember that Mr has no unit.
What is a mole?
A mole is a counting amount of particles.
Chemists use moles because atoms, molecules and ions are too small to count one by one.
Moles link the particle world to the mass we can measure in the lab.
Keep this conceptual, not historical. The key idea is that moles let chemists connect microscopic particles to macroscopic mass.
The mole equation
mass = moles × Mr
This can be rearranged to:
moles = mass ÷ Mr
Mr = mass ÷ moles
Use the exact equation forms the class will need later. Keep vocabulary consistent: mass in grams, moles in mol, Mr with no unit.
Choosing the right calculation
If you know mass and Mr, divide to find moles
If you know moles and Mr, multiply to find mass
If you know mass and moles, divide to find Mr
Start by writing the equation before using the calculator
Students often choose the wrong operation because they start with the numbers, not the equation. Encourage them to identify the missing quantity first.
Units in mole calculations
mass is measured in g
moles are measured in mol
Mr has no unit
Always give the final answer with the correct unit if it has one
If students write 'moles' instead of 'mol', correct this as notation practice. This slide prepares them for exam-style marks where units matter.
A guided mole calculation
Example: find the number of moles in 9 g of water.
Calculate Mr of H₂O = 18
Use moles = mass ÷ Mr
moles = 9 ÷ 18 = 0.5 mol
Slow the process down into named steps: formula, Mr, equation, substitution, answer. This structure will support weaker students in later examples.
From formula to answer
A complete mole calculation usually follows this route:
formula → Mr → equation → substitution → answer
Each step depends on the one before it being correct.
This is the lesson's core process slide. Refer back to it whenever students get stuck, rather than reteaching each problem as a new method.
If you know mass and Mr, how do you find moles?
mass + Mr
mass × Mr
mass ÷ Mr
Mr ÷ mass
Targets the key rearrangement before students attempt guided calculations.
Complete the method
A mole links particles to measurable mass in the lab. For 9 g of water, first find the Mr of H₂O, then use moles = mass ÷ Mr. A complete calculation follows formula → Mr → equation → substitution → answer.
Reinforces the core route on slide 11 and the worked example on slide 10.
Why write formula → Mr → equation before calculating?
Expected elements: identify the formula first, calculate Mr accurately, choose the correct rearranged equation based on the missing quantity, substitute values, and include the correct final unit where needed.
Finding mass from moles
Example: find the mass of 0.25 mol of CO₂.
Mr of CO₂ = 44
Use mass = moles × Mr
mass = 0.25 × 44 = 11 g
Contrast this with the previous example so students see that the formula changes with the missing quantity. Highlight that the method is similar but the operation is different.
Finding Mr from mass and moles
Example: a sample has a mass of 17 g and amount of 0.25 mol.
Use Mr = mass ÷ moles
Mr = 17 ÷ 0.25 = 68
This completes the triangle of rearrangements. Students should recognise that the same relationship can give three different unknowns.
Practical-style mass example
A practical might ask for a mass to weigh out.
If you need 0.10 mol of sodium chloride, NaCl:
Mr = 23 + 35.5 = 58.5
mass = 0.10 × 58.5 = 5.85 g
Link this to lab practice: balances measure mass, so chemists often convert a target amount in moles into grams before starting an experiment.
Another practical-style example
How many moles are in 24.5 g of H₂SO₄?
Mr = (2 × 1) + 32.1 + (4 × 16) = 98.1
moles = 24.5 ÷ 98.1
moles = 0.25 mol
This example combines accurate Mr calculation with the mole equation. It is a good place to remind students to keep enough calculator digits before rounding.
Common calculation errors
Using the wrong equation rearrangement
Forgetting to multiply subscripts in the formula
Giving grams for Mr
Rounding too early
Missing the unit g or mol in the final answer
Frame these as avoidable process errors rather than lack of ability. A neat line-by-line method is usually the best fix.
What is the mass of 0.25 mol of CO₂?
44 g
11 g
0.25 g
176 g
Uses the worked example pattern from slide 12. Distractors reflect common errors: copying Mr, copying moles, or multiplying by 4 instead of by 0.25.
Complete the practical summary
In a practical, balances measure mass, so chemists may convert moles into grams first. For NaCl, use mass = moles × Mr to find 5.85 g for 0.10 mol. Final answers should include g or mol when a unit is needed.
Builds on the practical-style examples and the common error slide: correct equation choice, practical meaning, and correct units.
What is one thing you learned about mole calculations?
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Which step in the method helped you most today?
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What part of these calculations still feels hardest?
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Glossary
Ar - relative atomic mass of an element compared with carbon-12
Mr - relative formula mass found by adding the Ar values in a formula
mole - a counting amount used for particles in chemistry
mass - the amount of substance measured in grams
formula - symbols showing which atoms are in a substance
subscript - the small number showing how many atoms of an element are present
rearrange - change the subject of an equation to find a different quantity
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