Chemical Calculations
Formulae, ionic equations, relative masses, moles, limiting reactants, gas volumes and solution concentration.
The lessons below are in teaching order. Work through them in turn, then come back after practice and use the outcomes on this page to choose what to review. Finishing a topic records participation only. Check your progress to see what you can answer independently and what to review next.
Lessons in order
- Chemical Formulae and Equations
Write formulae, balance equations without changing formulae, add state symbols and form ionic equations by cancelling spectators.
- Relative Mass and the Mole
Calculate relative masses, distinguish them from molar mass and convert mass and amount with units and magnitude checks.
- Stoichiometric Reacting Quantities
Establish a balanced equation, convert a supplied quantity to moles, apply its coefficient ratio and convert to the requested quantity.
- Limiting and Excess Reactants
Compare available amounts with stoichiometric requirements, identify the limiting reactant and calculate maximum product and excess remaining.
- Gas-Volume Calculations
Use a stated molar gas volume and conditions, convert cm³ and dm³, and connect gas amount to a balanced-equation ratio.
- Solution Concentration
Use amount concentration with volume in dm³ in both directions and combine it with a balanced-equation ratio where required.
Learning goals
- state the symbols of the elements and formulae of the compounds mentioned in the syllabus
- deduce the formulae of simple compounds from the relative numbers of atoms present and vice versa
- deduce the formulae of ionic compounds from the charges on the ions present and vice versa
- interpret chemical equations with state symbols
- construct chemical equations, with state symbols, including ionic equations.
- define relative atomic mass, Ar
- define relative molecular mass, Mr, and calculate relative molecular mass (and relative formula mass) as the sum of relative atomic masses
- define the term mole in terms of the Avogadro constant
- calculate stoichiometric reacting masses and volumes of gases (one mole of gas occupies 24 dm3 at room temperature and pressure); calculations involving the idea of limiting reactants may be set (knowledge of the gas laws and the calculations of gaseous volumes at different temperatures and pressures are not required)
- apply the concept of solution concentration (in mol/dm3 or g/dm3) to process the results of volumetric experiments (e.g. titration) and to solve simple problems. (appropriate guidance will be provided where unfamiliar reactions such as redox are involved. Calculations on % yield and % purity are not required.)