Chemical Formulae, Equations and Quantitative Chemistry
This domain tests whether you can write formulae, balance equations and calculate amounts using moles. Expect questions on relative formula mass, empirical formula, reacting masses, concentration and percentage yield.
Formulae and equations
Use valency to write formulae: the total positive valency must equal the total negative valency. Balance equations by adjusting coefficients only, never subscripts, so that the number of each type of atom is the same on both sides.
The mole and calculations
- Relative formula mass (Mr) is the sum of relative atomic masses in the formula.
- Moles = mass ÷ Mr.
- Use the balanced equation to find the mole ratio between substances.
- Concentration in mol/dm³ = moles ÷ volume in dm³. Convert cm³ to dm³ by dividing by 1000.
- Empirical formula is the simplest whole-number ratio of atoms; molecular formula is a whole-number multiple of it.
Worked example
4.8 g of magnesium burns completely in oxygen. Calculate the mass of magnesium oxide formed. (Ar: Mg = 24, O = 16) Equation: 2Mg + O₂ → 2MgO. Moles of Mg = 4.8 ÷ 24 = 0.20 mol. The equation shows 2 mol Mg give 2 mol MgO, so 0.20 mol Mg gives 0.20 mol MgO. Mr of MgO = 24 + 16 = 40. Mass of MgO = 0.20 × 40 = 8.0 g. The oxygen needed is 0.10 mol, which is 0.10 × 32 = 3.2 g; this checks with the law of conservation of mass.
Traps that cost marks
- Comparing masses directly instead of converting to moles.
- Forgetting to balance the equation before using the mole ratio.
- Using 35 instead of 35.5 for chlorine.
- Confusing empirical and molecular formulae.
- Forgetting to convert cm³ to dm³ in concentration calculations.