Determining Concentrations (Edexcel IAL Chemistry)

Core Practical 3: Hydrochloric Acid Concentration

Spec point — 8.21

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

Worked Example 25.0 cm³ of hydrochloric acid was titrated with a 0.200 mol dm^-3 solution of sodium hydrogencarbonate, NaHCO₃.

NaHCO₃+ HCl → NaCl + H₂O + CO₂

Use the following results to calculate the concentration of the acid, to 3 significant figures.

Answer

Step 1: Calculate the average titre

• Average titre$=\frac{22.80+22.80}{2}=$22.80 cm³

Step 2: Calculate the number of moles of sodium hydrogencarbonate

• Moles = $\frac{22.80}{1000}$ 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 = $=\frac{moles}{volume}=\frac{4.56\times10^{-3}}{(25.0/1000)}=$0.182 mol dm^-3