Chemical reactions, formulae, equations and quantitative chemistry
Chemical reactions, formulae, equations and quantitative chemistry tests conservation of mass, formula writing, balancing and simple calculations.
Core rules
- Word equations name reactants and products. Symbol equations must be balanced: same number of each atom on both sides.
- Diatomic elements: H2, N2, O2, F2, Cl2, Br2, I2.
- State symbols: (s), (l), (g), (aq).
- Relative formula mass Mr is the sum of relative atomic masses.
- Conservation of mass means total mass of reactants equals total mass of products, allowing for gases escaping.
- For a reaction, moles = mass / Mr. Use the balanced equation ratio to find unknown masses.
- Empirical formula is the simplest whole-number ratio; molecular formula is the actual numbers.
- Percentage yield = actual yield / theoretical yield ×100. The limiting reactant is used up first and fixes the maximum product.
Worked example
Methane burns: CH4 + 2O2 -> CO2 + 2H2O. Balance: C1, H4, O4 on both sides. Calculate the mass of CO2 formed from 8.0 g CH4. Mr(CH4)=12+4=16; moles CH4=8.0/16=0.50 mol. Ratio CH4:CO2 =1:1, so 0.50 mol CO2. Mr(CO2)=44; mass=0.50×44=22 g. The mass of oxygen used would be 1.0 mol O2, 32 g.
Traps
Balancing by changing subscripts; forgetting O2 is diatomic; using mass ratio directly instead of mole ratio; ignoring limiting reactant; leaving state symbols off when asked.