Titrations (AQA A Level Chemistry)

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Titrations

  • Volumetric analysis is a process that uses the volume and concentration of one chemical reactant (a standard solution) to determine the concentration of another unknown solution
  • The technique most commonly used is a titration
  • The volumes are measured using two precise pieces of equipment, a volumetric or graduated pipette and a burette

Pipettes and burette, downloadable IB Chemistry revision notes

Equipment used to measure volumes precisely in titrations

  • Burettes are usually marked to a precision of 0.10 cm3
    • Since they are analogue instruments, the uncertainty is recorded to half the smallest marking, in other words to ±0.05 cm3

  • The stoichiometric point or equivalence point occurs when the two solutions have reacted completely and is shown with the use of an indicator

Titration, downloadable IB Chemistry revision notes

The steps in a titration

  • The steps in a titration are:
    • Measuring a known volume (usually 20 or 25 cm3) of one of the solutions with a volumetric or graduated pipette and placing it into a conical flask
    • The other solution is placed in the burette
    • A few drops of the indicator are added
    • The tap on the burette is carefully opened and the solution added, portion by portion, to the conical flask until the indicator just changes colour(this is the end point)
    • Multiple trials are carried out until concordant results are obtained

 

Titration Calculations

  • Titration calculations are used to find the concentration of unknown solutions
  • They can also be used to calculate the pH after a given point during a titration

Worked example

Example 1: Calculations from titration results

In a titration, 25.00 cm3 of 0.05 mol dm-3 hydrochloric acid was neutralised by 8.50 cm3 of sodium hydroxide solution. Calculate the concentration of the sodium hydroxide solution.

Answer

Step 1: Find the number of moles of acid

moles of acid = concentration x volume in dm3

moles of acid = 0.05 x 25/1000 = 1.25 x 10-3 mol

Step 2: Deduce the number of moles of alkali

The equation for the reaction shows the mole ratio is 1:1

HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l)

∴ moles of alkali  = 1.25 x 10-3 mol

Step 3: Work out the concentration of the alkali

concentration = moles/volume in dm3

concentration = 1.25 x 10-3/0.0085 = 0.15 mol dm-3

Worked example

Example 2: Calculating the pH in a strong acid-strong base titration

50.0 cm3 of 0.10 mol dm3 NaOH is gradually added to 25.0 cm3 of 0.15 mol dm3 hydrochloric acid. Determine the pH after 45 cm3 of NaOH has been added. (Kw = 1 x 10-14 mol2 dm-6 at 298 K).

Answer

Step 1: Find the number of moles of acid

moles of acid = concentration x volume in dm3

moles of acid = 0.15 x 25/1000 = 3.75 x 10-3 mol

Step 2: Deduce the number of moles of alkali added

The equation for the reaction shows the mole ratio is 1:1

HCl (aq) + NaOH (aq)  NaCl (aq) + H2O (l)

moles of alkali added = 0.10 x 45/1000 =  4.50 x 10-3 mol

∴ moles of alkali in excess = (4.50 x 10-3- 3.75 x 10-3) = 7.5 x 10-4 mol

Step 3: Work out the concentration of the alkali

concentration = moles/volume in dm3

concentration = 7.5 x 10-4/0.070 = 0.0107 mol dm-3

Step 4: Use Kw to find the concentration of H+

Kw = [H+][OH-]

[H+] = Kw /[OH-] = 1.00 x 10-14/0.0107 = 9.35 x 10-13

Step 5: Find the pH

-log[H+] = -log(9.35 x 10-13)

pH = 12.03

Examiner Tip

You will be asked to perform calculations only on monoprotic acids. A monoprotic acid has only one acidic hydrogen, like HCl. Sulfuric acid, H2SO4, is a diprotic acid.

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Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.