Moles, Formulae and Stoichiometry
Amount of substance, Avogadro's constant, empirical and molecular formulae, reacting masses, gases, solutions, yields and atom economy.
How to study A-level Chemistry
Represent the species and process correctly, conserve atoms, charge and energy, calculate with units, then test the result against observations and chemical evidence.
Core concepts
Concept 1
Balanced equations give mole ratios that connect measured mass, gas volume, solution concentration and particle number.
Exam cue: Convert every given quantity to moles before applying the equation ratio.
Concept 2
Empirical formula is the simplest whole-number ratio; molecular formula also requires relative molecular mass.
Exam cue: Use SI units in the ideal-gas equation and show unit conversions explicitly.
Concept 3
Yield, purity and atom economy answer different questions about process outcome and resource use.
Exam cue: Identify limiting reagent before calculating theoretical yield.
Risk pitfalls and guardrails
Using mass ratios directly where the equation gives mole ratios.
Guardrail: Do not substitute a memorised equation, mechanism or trend until you have identified the species, conditions and chemical model required.
Mixing cm³ and dm³ in concentration calculations.
Guardrail: Do not substitute a memorised equation, mechanism or trend until you have identified the species, conditions and chemical model required.
Confusing percentage yield with atom economy.
Guardrail: Do not substitute a memorised equation, mechanism or trend until you have identified the species, conditions and chemical model required.
Memory anchors
Amount of Substance
Amount in moles equals number of entities divided by Avogadro's constant.
Mass and Moles
Amount in moles equals mass divided by molar mass.
Solution Amount
Amount in moles equals concentration multiplied by volume in dm³.
Ideal Gas Equation
For an ideal gas, pV = nRT using compatible SI units.
Atom Economy
Atom economy is desired-product formula mass divided by total reactant formula mass, multiplied by 100.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
What amount of NaCl is present in 5.85 g? Use M(NaCl) = 58.5 g mol⁻¹.
What mass of CaCO₃ corresponds to 0.0750 mol? Use M(CaCO₃) = 100.1 g mol⁻¹.
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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