Chemistry worked examples
Reading a worked solution isn't revising it. Each example fades in three stages: see it fully worked, then fill in the blanked middle steps, then do the whole thing with just the question. Each one names its 8462 specification point and tier, and flags the slip that most often costs the mark.
Quantitative chemistry
Balancing a symbol equation with state symbols
Write the formulae first, balance with the big numbers only, check every atom, then add (s), (l), (g) and (aq).
Relative formula mass with brackets
Multiply out the bracket before you add — the formula mass every mole calculation starts from.
Moles from mass, then number of molecules
Mass ÷ relative formula mass gives moles; moles × the Avogadro constant gives particles. Round only at the end.
Reacting masses with a limiting reactant
Turn both masses into moles, use the equation's ratio to find which runs out, then work from that one.
Concentration in g/dm³ and mol/dm³
Convert cm³ to dm³ first; mass ÷ volume gives g/dm³, moles ÷ volume gives mol/dm³ (the mol/dm³ half is Higher, chemistry only).
Percentage yield from a reacting mass
Find the theoretical mass from the equation, then actual ÷ theoretical × 100 (chemistry only).
Atom economy
Relative formula mass of the desired product (balancing number included) ÷ total relative formula mass of the reactants × 100 (chemistry only).
Titration calculation
Concordant mean titre, moles of the known solution, the equation's ratio, then divide by the unknown's volume (chemistry only).
Volume of gas at room temperature and pressure
Moles of gas × 24 dm³ at RTP — then convert to the unit the question asks for (chemistry only).
Chemical changes
Energy changes
Rate and extent of chemical change
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