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

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

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

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

Evaluation

• Calculate mean titre using concordant results 

• Mean titre =  = 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