Chemistry Lab · Free lesson · 24 minutes
The mole connects a balance to particles
Calculate a compound's molar mass, convert between grams and moles with units and distinguish amount from a particle count.
SS2 · amount of substance01 · Think first
Start with a question.
Equal masses of water and carbon dioxide do not contain equal numbers of molecules. A mole counts specified entities; molar mass connects that count to what a balance measures.
What you already know
- Read formula subscripts
- Multiply and divide decimals
- Distinguish g from mol
02 · See it
Convert through chemical amount
Use the correct molar mass for the specified substance. The units show why mass is divided, rather than multiplied, to find moles.
- Mass m
- grams (g)
- Divide by M
- grams per mole (g mol⁻¹)
- Amount n = m/M
- moles (mol)
- Multiply by Nₐ
- specified particle count
Use the justified worked steps below to explore this idea.
03 · Show steps · explain why
One justified step at a time.
- 1
For H₂O, M = 2(1) + 16 = 18 g mol⁻¹
The formula fixes the composition. These rounded atomic masses are supplied for this lesson; use the values given in an examination.
04 · Mistake clinic
Find the first wrong idea.
Moles in 9 g of water = 9 × 18 = 162 mol.
Compare the correction and its reason
n = 9/18 = 0.50 mol.
Multiplication gives the wrong units. Also 9 g is less than the 18 g mass of one mole, so the amount must be less than one mole.
Check: Estimate against one mole, then check the units. Do not divide grams directly by Avogadro's constant.
A mistake is evidence about the next idea to repair. It is not a label for the learner.
05 · Your turn
Try first. Then use a hint.
06 · Check & reflect → next step
Understanding includes explaining.
What entity does your mole count specify? Explain one conversion with units before using a formula.
Combine amounts with a balanced reaction to reason about neutralisation; a chemical ratio comes before any calculation.
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JEM-CH-C06-002 · Phase 5B Chemical Language Mole Stoichiometry Solutions Quantitative Analysis · Lab 06 · printed pp. 22–24
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