Formulae, Equations and the Mole Concept
Core ideas
A chemical formula shows the ratio of atoms in a compound. A word equation names reactants and products; a symbol equation uses formulas and must be balanced so atoms are conserved. State symbols (s), (l), (g), (aq) may be required.
The mole is the amount of substance containing 6.02 × 10²³ particles (Avogadro constant). Key relationships: mass = moles × molar mass; moles of gas = volume / 24 dm³ at room temperature and pressure; concentration = moles / volume in dm³. Relative formula mass is the sum of relative atomic masses in the formula. Percentage yield and empirical formula calculations are common.
Traps to watch
- Always balance equations before using mole ratios; the coefficients give the reacting ratio.
- Convert cm³ to dm³ by dividing by 1000.
- Percentage yield is actual/theoretical × 100, not the reverse.
- Empirical formula uses the simplest whole-number ratio, not molecular formula.
Rule of thumb
Use the mole triangle: cover what you want. mass = moles × Mr; moles = mass ÷ Mr; moles = concentration × volume (dm³).
Worked example
Calculate the mass of 0.25 mol of CaCO₃ (Mr = 100). Mass = 0.25 × 100 = 25 g.