IGCSEScienceOxford AQACombined Science Double AwardChemistryRevision NotesQuantitative ChemistryUsing Molar Concentrations of Solutions & Amount of Substance in Relation to Volumes of GasesConcentration of an Unknown acid

Concentration of an Unknown acid (Oxford AQA IGCSE Combined Science Double Award)

Revision Note

Philippa Platt

Written by: Philippa Platt

Reviewed by: Stewart Hird

Oxford AQA Combined Science Double Award: ChemistryOxford AQA Chemistry

Required Practical: Concentration of an Unknown Acid

Objective

  • To determine the reacting volumes of a strong acid and a strong alkali by titration

  • To calculate the concentration of an unknown acid

Apparatus

  • 25 cm3 volumetric pipette

  • Pipette filler

  • 50 cm3 burette

  • 250 cm3 conical flask

  • Small funnel

  • 0.1 mol/dm3 sodium hydroxide solution

  • Sulfuric acid of unknown concentration

  • Methyl orange indicator

  • Clamp stand, clamp & white tile

Apparatus used in a titration

Diagram to show how to perform a titration requires a burette, conical flask and pipette
Performing a titration requires a burette, conical flask and pipette
A diagram to show a titration using a white tile to show the endpoint
Using a white tile to determine the endpoint

Method

  1. Use the pipette and pipette filler to place exactly 25 cm3 sodium hydroxide solution into the conical flask

  2. Clamp a burette vertically in the clamp stand

  3. Place the conical flask on a white tile so the tip of the burette is inside the flask

  4. Fill the burette with sulfuric acid and record the volume to the nearest 0.05 cm3

  5. Add a few drops of methyl orange to the solution in the conical flask

  6. Add the solution from the burette in 1 – 3 cm3 portions, while swirling the flask vigorously

  7. Quickly close the tap when the end-point is reached (from yellow to just red / orange)and record the volume, placing your eye level with the meniscus

    1. This is known as a rough titration

  8. Repeat the titration with a fresh batch of sodium hydroxide

  9. As the rough end-point volume is approached, add the solution from the burette one drop at a time until the indicator just changes colour

  10. Record the volume to the nearest 0.05 cm3

  11. Repeat until you achieve two concordant results (two results that are within 0.1 cm3 of each other) to increase accuracy

 

Rough Titre 

Titre 1 

Titre 2 

Final reading (cm3)

 

 

 

First reading  (cm3)

 

 

 

Titre  (cm3)

 

 

 


Practical Tip

  • Use a funnel to fill the burette but be sure to remove it before starting the practical as it can drip liquid into the burette, making the initial reading false

  • Make sure the tap is closed before adding the acid to the burette

  • Once full, run through a small portion of acid in the burette to remove any air bubbles

Results

Table of results

 

Rough Titre 

Titre 1 

Titre 2 

Final reading (cm3)

16.00

14.90

15.20

First reading  (cm3)

0.10

0.00

0.20

Titre  (cm3)

15.90

14.90

15.00

Evaluation

  • Calculate mean titre using concordant results 

  • Mean titre = fraction numerator 14.90 space plus space 15.00 over denominator 2 end fraction = 14.95 cm3

  • The mean titre is used to determine the relative concentrations of both solutions

  • 14.95 cm3 of sulfuric was required to neutralise 25 cm3 0.1 mol/dm3 sodium hydroxide therefore the acid is more concentrated than the alkali

Worked Example

A student titrated 25 cm3 of 0.5 mol/dm3 sodium hydroxide against sulfuric acid with an unknown concentration.

She recorded her results in the following table:

 

Rough Titre 

Titre 1 

Titre 2 

Final reading (cm3)

30.05

29.15

30.50

First reading  (cm3)

0.10

0.05

1.50

Titre  (cm3)

 

 

 

i) Complete the table.

ii) Use your answer to part( i) to calculate the mean titre in cm3.

Answer:

i) The completed table is:

 

Rough Titre 

Titre 1 

Titre 2 

Final reading (cm3)

30.05

29.15

30.50

First reading  (cm3)

0.10

0.05

1.50

Titre  (cm3)

29.95

29.10

29.00

ii) To calculate the mean titre only concordant results must be used: 

Mean titre = fraction numerator 29.10 space plus space 29.00 over denominator 2 end fraction = 29.05 cm3

Examiner Tips and Tricks

Only concordant results should be used to calculate a mean titre, this means that they are within 0.1 cm3 of one another.

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    Philippa Platt

    Author: Philippa Platt

    Expertise: Chemistry

    Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener

    Stewart Hird

    Author: Stewart Hird

    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 Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.