Quantitative AnalysisEdexcel GCSE Chemistry: Revision notes

(Higher tier) Concentration can be expressed in mol dm⁻³ (moles of solute per dm³ of solution) as well as g dm⁻³ (grams per dm³).

concentration (mol dm⁻³) = moles of solute ÷ volume of solution (dm³)

To convert between g dm⁻³ and mol dm⁻³:

• g dm⁻³ → mol dm⁻³: divide by the relative formula mass (Mr) of the solute
• mol dm⁻³ → g dm⁻³: multiply by the relative formula mass (Mr) of the solute

Remember: cm³ must be converted to dm³ by dividing by 1000 before using these equations.

Core Practical: an accurate acid-alkali titration uses a burette, a pipette and a suitable indicator.

Method outline:

1. Use a pipette to measure an accurate, fixed volume of one solution (e.g. the alkali) into a conical flask, and add a few drops of indicator
2. Fill a burette with the other solution (e.g. the acid)
3. Add the acid from the burette to the flask, swirling constantly, until the indicator just changes colour (the end point) — record this volume
4. Repeat until you get concordant results (titres agreeing within 0.10 cm³) and calculate a mean, ignoring anomalous results

(Higher tier) Using titration results:

moles = concentration (mol dm⁻³) × volume (dm³)

Using the balanced equation, compare mole ratios between the two reactants to calculate the unknown concentration or unknown volume of a solution.

The theoretical yield is the maximum possible mass of product calculated from the balanced equation, assuming everything reacts perfectly. The actual yield obtained in practice is usually less than this, because of:

• Incomplete reactions (not all reactants convert to product)
• Practical losses during the experiment (e.g. product left on filter paper, spillages)
• Competing/unwanted side reactions producing other products

percentage yield = (actual yield ÷ theoretical yield) × 100

Atom economy measures what proportion of the mass of reactants ends up as the desired product, rather than being wasted in by-products:

atom economy = (relative formula mass of desired product ÷ sum of relative formula masses of all reactants) × 100

A reaction pathway may be chosen based on atom economy, yield, rate, equilibrium position and usefulness of by-products (higher tier) — a process with high atom economy wastes less raw material and is generally more sustainable and cost-effective, even if its rate is slower.

(Higher tier) Molar gas volume: one mole of molecules of any gas occupies the same volume at room temperature and pressure (RTP) — the molar volume, given in exams as 24 dm³ (24,000 cm³). This, together with balanced equations, can be used to calculate:

• The mass of a solid reacting with or produced alongside a given volume of gas
• Volumes of gases involved in a gaseous reaction, using Avogadro's law (equal moles of any gas occupy equal volumes at the same temperature and pressure) and the mole ratios from the balanced equation