Determining Concentrations (Edexcel International AS Chemistry)

Performing the Titration

• The key piece of equipment used in the titration is the burette
• 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 end point or equivalence point occurs when the two solutions have reacted completely and is shown with the use of an indicator

The steps in a titration

• A white tile is placed under the conical flask while the titration is performed, to make it easier to see the colour change

Titrating 

• The steps in a titration are:
  • Measuring a known volume (usually 20 or 25 cm³) of one of the solutions with a volumetric pipette and placing it into a conical flask
  • The other solution is placed in the burette
    • To start with, the burette will usually be filled to 0.00 cm³

  • A few drops of the indicator are added to the solution in the conical flask
  • The tap on the burette is carefully opened and the solution added, portion by portion, to the conical flask until the indicator starts to change colour
  • As you start getting near to the end point, the flow of the burette should be slowed right down so that the solution is added dropwise
    • You should be able to close the tap on the burette after one drop has caused the colour change

  • Multiple runs are carried out until concordant results are obtained
    • Concordant results are within 0.1 cm³ of each other

Recording and processing titration results

• Both the initial and final burette readings should be recorded and shown to a precision of  ±0.05 cm³, the same as the uncertainty

A typical layout and set of titration results

• The volume delivered (titre) is calculated and recorded to an uncertainty of ±0.10 cm³
  • The uncertainty is doubled, because two burette readings are made to obtain the titre (V final – V initial), following the rules for propagation of uncertainties

• Concordant results are then averaged, and non-concordant results are discarded
• The appropriate calculations are then done

Answer

Step 1: Calculate the average titre

• • Average titre22.80 cm³

Step 2: Calculate the number of moles of sodium hydrogencarbonate

• • Moles =  x 0.200 = 4.56 x 10^-3 moles

Step 3: Calculate (or deduce) the number of moles of hydrochloric acid

• • The stoichiometry of NaHCO₃ : HCl is 1 : 1
  • Therefore, the number of moles of sodium hydrogencarbonate is also 4.56 x 10^-3 moles

Step 4: Calculate the concentration of hydrochloric acid

• • Concentration = 0.182 mol dm^-3