Calculations
Calculations are where the reliable marks are — and where a missed unit conversion or mole ratio throws them away. Each page gives the key relationship, then a worked exam question you do on paper and mark against a real mark scheme.
GCSE (AQA 8462)
Moles from mass
n = m / Mr
n = m / Mr underpins nearly every calculation — get this automatic and the rest follow.
Titration concentration
moles = conc x volume
Two easy marks are lost every year to the cm^3 -> dm^3 conversion. Do the acid side first.
Reacting masses
mass → moles → ratio → mass
Mass doesn't go straight to mass — you cross the 'moles bridge' using the balanced equation.
Percentage yield
actual / theoretical x 100
Actual over theoretical — and a yield above 100% always means an arithmetic slip.
Atom economy
Mr(product) / Mr(reactants) x 100
It's about the balanced equation, not the experiment — desired product over the reactant mass.
A-level (AQA 7405)
Equilibrium constant Kc
Kc = [products] / [reactants]
Products over reactants, each raised to its balancing number — and the units come from the powers.
pH of a strong acid
pH = -log10[H+]
For a strong monoprotic acid [H+] equals the acid concentration — then it's just -log.
Enthalpy change from calorimetry
q = m c dT ; dH = -q / n
q = m c dT gives the energy; divide by the moles reacted and flip the sign for exothermic.
The ideal gas equation
pV = nRT
pV = nRT only works in SI units — pascals, cubic metres and kelvin — so convert first.
Master the method, not just the answer
Tell me which calculation you keep dropping marks on — I'll drill the method until it's automatic. Your first lesson is free.