A level Chemistry · Edexcel 9CH0 · Topic 5B
Concentrations and titrations
Key points
- Concentration in mol dm⁻³ (V in dm³; divide cm³ by 1000). Mass concentration is in g dm⁻³.
- Make a standard solution: weigh solid, dissolve, transfer with washings to a volumetric flask, make up to the mark and invert to mix.
- In a titration use a pipette for a fixed volume, a burette for the titrant, and an indicator; repeat to get concordant titres (within 0.10 cm³) and average them.
- Steps: moles of known → mole ratio → moles of unknown → concentration or mass.
- Uncertainty: % uncertainty = (uncertainty ÷ value) × 100; a burette reading is ±0.05 cm³, so a titre has ±0.10 cm³.
- Parts per million (ppm) is used for very dilute solutions or gases.
- Common errors: not rinsing the burette with the titrant, air bubble below the tap, reading the top rather than bottom of the meniscus.
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Most reactions you carry out in the lab happen in solution, so you need to move confidently between concentration, volume and moles. Titrations then let you measure an unknown concentration very precisely, and they are the basis of Core…
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