Formulae, Equations, Moles and Reacting Quantities
Write chemical formulae and balanced equations, calculate gram formula mass, moles, solution quantities, reacting masses and percentage composition.
How to study National 5 Chemistry
Move between particle explanations, formulae, calculations, experimental evidence and justified conclusions, while using official materials for exact written-paper and assignment practice.
Core concepts
Concept 1
Names, valencies, prefixes, Roman numerals and group-ion data determine chemical formulae; subscripts show particle ratios while superscripts show charge.
Exam cue: Write correct formulae before balancing the equation; do not alter subscripts to balance it.
Concept 2
Balanced equations conserve each element and can include state symbols; coefficients provide mole ratios between reacting substances.
Exam cue: Convert solution volume to litres before using molar concentration.
Concept 3
Gram formula mass links mass and amount in moles, while solution amount links concentration and volume in litres.
Exam cue: Use the balanced-equation ratio between substances before converting back to the requested unit.
Risk pitfalls and guardrails
Confusing a coefficient with a subscript.
Guardrail: Check formulae, charges, units, state or experimental conditions before committing to the final conclusion.
Using cubic centimetres directly in a moles-per-litre relationship.
Guardrail: Check formulae, charges, units, state or experimental conditions before committing to the final conclusion.
Calculating a gram formula mass but forgetting the stoichiometric ratio.
Guardrail: Check formulae, charges, units, state or experimental conditions before committing to the final conclusion.
Memory anchors
Chemical formula
A formula identifies the elements or ions and their simplest required ratio.
Balanced equation
Only coefficients change when balancing; correct chemical formulae stay fixed.
Gram formula mass
Gram formula mass is found by adding the relative atomic masses in the formula.
Mass and moles
Number of moles equals mass divided by gram formula mass.
Solution moles
Number of moles equals concentration in mol l⁻¹ multiplied by volume in litres.
Percentage composition
Element percentage equals its formula-mass contribution divided by total 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 is the formula of magnesium chloride?
What is the formula of aluminium oxide?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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