Determining Concentrations (Edexcel A Level Chemistry)

Revision Note

Richard

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Richard

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Core Practical 3: Hydrochloric Acid Concentration

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

Titration, downloadable IB Chemistry revision notes

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

Titration apparatus, downloadable AS & A Level Chemistry revision notes

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

Titration results, downloadable IB Chemistry revision notes

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.

nrhOgeQ__example-titration-data-for-calculation

Answer

Step 1: Calculate the average titre

    • Average titreequals space fraction numerator 22.80 space plus space 22.80 over denominator 2 end fraction space equals22.80 cm3

Step 2: Calculate the number of moles of sodium hydrogencarbonate

    • Moles = fraction numerator 22.80 over denominator 1000 end fraction 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 = equals space fraction numerator m o l e s over denominator v o l u m e end fraction space equals fraction numerator 4.56 space cross times space 10 to the power of negative 3 end exponent over denominator left parenthesis 25.0 space divided by space 1000 right parenthesis end fraction space equals space0.182 mol dm-3 

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.