Monitoring reactions: moles, concentration and titrations (Higher)

The mole

One mole = 6.02 × 10²³ particles (Avogadro's constant). The mass of one mole of a substance, in grams, equals its relative formula mass (Mr). Example: 1 mol H₂O = 18 g; 1 mol NaCl = 58.5 g.

n = mass / Mr n = c × V (V in dm³, c in mol/dm³)

Concentration

Concentration is amount of solute per unit volume. Two units appear in OCR papers:

• mass concentration: g/dm³
• molar concentration: mol/dm³

To convert: c (mol/dm³) = c (g/dm³) ÷ Mr.

Remember: 1 dm³ = 1000 cm³ = 1 litre.

Titration (PAG C4)

Titration finds the exact volume of one solution that reacts with a known volume of another, used here for acid–alkali neutralisation.

1. Pipette 25.0 cm³ of alkali into a conical flask, add 2–3 drops phenolphthalein (pink in alkali).
2. Fill burette with acid, take initial reading to 0.05 cm³.
3. Add acid, swirling, until the colour just disappears — this is the end-point.
4. Repeat until at least two concordant titres (within 0.10–0.20 cm³).
5. Use the mean concordant titre.

OCR exam tip

Always state significant figures and units. Common loss-of-mark errors: forgetting to convert cm³ → dm³, using the unrounded value once and the rounded value later, or quoting an answer to 4 s.f. when only 3 are justified.