The mole concept, without the fear
The mole is just a counting unit for particles — the chemist's “dozen.” Once you can convert between moles and mass, concentration or gas volume, most G3 Chemistry calculations become the same three steps: find moles, use the equation's ratio, convert back.
Key facts
| Topic | Chemical Calculations |
| Key relation | moles = mass ÷ Mr |
| Solutions | moles = volume (dm³) × concentration |
| Gases at rtp | 1 mole = 24 dm³ |
What a mole actually is
A mole is a fixed number of particles (atoms, molecules or ions) — Avogadro's number. You never count them directly; instead you measure something you can (mass, volume of solution, or volume of gas) and convert. That conversion is the whole skill.
The three conversions
From mass: moles = mass ÷ relative formula mass (Mr). From a solution: moles = volume in dm³ × concentration in mol/dm³. From a gas: at room temperature and pressure, one mole occupies 24 dm³, so moles = volume ÷ 24. That 24 dm³ figure is the only gas-volume assumption K324 needs — gas-law calculations are not in the syllabus.Mole ratios from equations
A balanced equation is a recipe in moles. If the equation says 1 mole of A makes 2 moles of B, then the moles of B are twice the moles of A. The standard problem is: convert the quantity you're given into moles, multiply by the ratio from the equation, then convert that answer back into mass, volume or concentration.
Common mistakes
The usual slips: mixing up Ar (an atom) with Mr (a formula); forgetting to convert cm³ to dm³ (divide by 1000); using an unbalanced equation, so the ratio is wrong; and dropping units or significant figures, which K324 penalises. Write the units at every step — they catch most errors before they cost marks.
Frequently asked questions
What is a mole in chemistry?
A mole is a fixed number of particles — a counting unit. You measure mass, solution volume or gas volume and convert to moles to do calculations.
How do you find moles of a gas?
At room temperature and pressure, one mole of any gas occupies 24 dm³, so moles = volume ÷ 24. This is the only gas-volume assumption K324 requires.
What's the difference between Ar and Mr?
Ar is the relative atomic mass of one element; Mr is the relative formula mass of a compound (the Ar values added up). Use Mr for compounds.
Why do my mole answers come out wrong?
Most often: cm³ not converted to dm³, an unbalanced equation giving the wrong ratio, or Ar/Mr confusion. Writing units at each step catches these.
G3 Chemistry, made clear
LearnUp teaches K324 Chemistry topic by topic, with practical drills and exam-format questions. Explore the programme on BrainBuzz.
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