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Concentration of an Unknown AcidOxford AQA IGCSE Chemistry: Revision notes

Section 1

What is titration used for?

Titration is a technique used to find the volume of one solution that exactly reacts with a known volume of another. It can be used to find the concentration of an unknown acid by reacting it with an alkali of known concentration (or vice versa).

  • This platform's required practical: establish the concentration of an unknown strong acid through titration with a strong base
  • Only strong acids and strong alkalis are used: hydrochloric, sulfuric and nitric acids
  • A suitable indicator is used to show exactly when the reaction is complete (the end point)
Key termstitrationend point

Section 2

How is a titration carried out?

  1. A known volume of alkali (measured accurately using a pipette) is placed in a conical flask, with a few drops of indicator added
  2. The unknown acid is added from a burette, a small amount at a time, swirling the flask constantly
  3. The acid is added until the indicator just changes colour, showing the reaction is complete - this is the end point
  4. The volume of acid added is recorded; the titration is usually repeated until concordant results (within 0.10 cm3 of each other) are obtained, and a mean (average) volume calculated, ignoring anomalous results
Key termsburetteconcordant results
Exam tip

Examiners reward mentioning a rough titration first, followed by accurate repeats, and taking the mean of concordant titres only, ignoring any anomalous result.

Section 3

How do you calculate the concentration of the unknown acid?

Once the volume of acid needed to exactly neutralise the alkali is known, the concentration of the acid can be calculated:

  1. Use the balanced symbol equation to find the mole ratio between acid and alkali
  2. Calculate the number of moles of alkali used: n = concentration x volume (in dm3)
  3. Use the mole ratio to find the number of moles of acid that reacted
  4. Calculate the concentration of the acid: concentration (mol/dm3) = number of moles / volume of acid used (dm3)
  5. If needed, convert the concentration to g/dm3 by multiplying by the acid's relative formula mass (Mr)
Example

25.0 cm3 of 0.100 mol/dm3 NaOH exactly neutralises 22.5 cm3 of hydrochloric acid (1:1 ratio, HCl + NaOH -> NaCl + H2O). Moles NaOH = 0.100 x (25.0/1000) = 0.00250 mol. Moles HCl = 0.00250 mol (1:1 ratio). Concentration of HCl = 0.00250 / (22.5/1000) = 0.111 mol/dm3.

Must Know

  • Titration finds the volume of acid or alkali needed to exactly react with a known volume of the other, using a suitable indicator to show the end point
  • Only strong acids (hydrochloric, sulfuric, nitric) and strong alkalis are used
  • Repeat titrations until concordant results (within 0.10 cm3) are obtained and calculate a mean, ignoring anomalies
  • Moles = concentration x volume (dm3); use the mole ratio from the balanced equation to link acid and alkali moles
  • Concentration (mol/dm3) = moles / volume (dm3); multiply by Mr to convert to g/dm3

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