Volumetric Solutions & Analysis (Oxford AQA International A Level (IAL) Chemistry): Revision Note

Required Practical 1: Volumetric Solutions & Acid-Base Titration

Required practical 1 is split into:

• Part A: Make up a volumetric solution

• Part B: Carry out a simple acid-base titration

Part A: Make up a volumetric solution

Objective

To prepare 250 cm³ of 0.100 mol dm^-3 sodium hydrogensulfate solution, NaHSO₄ (aq).

Apparatus

• Weighing bottle

• 250 cm³ volumetric flask

• Solid sodium hydrogensulfate, NaHSO₄ (s)

  • M_r = 120.1 g mol^-1

• Filter funnel

• Spatula

• Deionised / distilled water

• 250 cm³ beaker

• Glass rod

• Digital mass balance (reading to 2 or 3 decimal places)

Method

• Calculate the mass of solid sodium hydrogensulfate required to make 250 cm³ of a 0.100 mol dm^-3 solution

  • 1 mol dm^-3 NaHSO₄ = 120.1 g in 1 dm³

  • 0.100 mol dm^-3 NaHSO₄ = 12.01 g in 1 dm³

  • 0.100 mol dm^-3 NaHSO₄ = 3.0225 g in 250 cm³

• Measure out 3.0225 g of solid sodium hydrogensulfate into the weighing bottle

  • The exact mass needs to be recorded because it is not possible to measure to this precision with the mass balances for this experiment

• Transfer the solid sodium hydrogensulfate from the weighing bottle to the beaker

• Rinse the weighing bottle with deionised water and add the washings to the beaker

• Add 100 cm³ of deionised water and stir until all of the solid dissolves

• Transfer the sodium hydrogen carbonate solution from the beaker to the volumetric flask

• Rinse the beaker and glass rod with deionised water and add the washings to the volumetric flask

• Make the volumetric flask up to the graduation mark

• Insert the stopper and shake / invert the volumetric flask to mix the contents

• Calculate the exact concentration of the sodium hydrogensulfate solution in mol dm^-3

Diagram

Practical Tip

• The most common source of error in the preparation of a standard solution is the loss of solid, when it is transferred from one vessel to another

Results

• The only result for this part of the required practical is the mass of sodium hydrogensulfate

  • e.g. mass of sodium hydrogensulfate = 3.02 g

• Remember: The exact mass of sodium hydrogensulfate is determined by the number of decimal places on the mass balance

Evaluation

• The exact concentration of the sodium hydrogensulfate solution is:

  • Mass of NaHSO₄ = 3.02g

  • Moles of NaHSO₄ = = 0.0251

  • Concentration =

  • Concentration = = 0.1004 mol dm^-3

Part B: Carry Out a Simple Acid-Base Titration

Objective

To determine the concentration of a solution of sodium hydroxide by titration using a sodium hydrogensulfate solution that has a known concentration.

Apparatus

• Burette, stand and clamp

• 25 cm³ volumetric pipette with pipette bulb / filler

• 250 cm³ conical flasks

• Funnel

• Deionised / distilled water wash bottle

• Phenolphthalein indicator

• 150 cm³ sodium hydrogensulfate (standard) solution

• 150 cm³ sodium hydroxide (unknown concentration) solution

Method

1. Rinse the burette with the sodium hydrogensulfate standard solution

2. Then fill the burette with the sodium hydrogensulfate standard solution

3. Rinse a 250 cm³ conical flask with deionised / distilled water

4. Rinse the 25 cm³ volumetric pipette with the sodium hydroxide solution

5. Use the volumetric pipette to transfer exactly 25.0 cm³ of sodium hydroxide solution into the rinsed 250 cm³ conical flask

6. Add two to three drops of phenolphthalein indicator to the solution in the conical flask

7. Construct a results table

8. Record the initial burette reading to the appropriate precision

9. Titrate the contents of the conical flask by adding sodium hydrogensulfate solution to it from the burette

   • Add the sodium hydrogensulfate solution slowly with swirling to mix the solution

   • Add the sodium hydrogensulfate solution dropwise near the end-point

   • At the end-point the indicator undergoes a definite colour change

10. Record the final burette reading in your results table

11. In your results table, calculate / record the volume of sodium hydrogensulfate solution used

12. Repeat the titration until you obtain two results which are concordant

    • Concordant results are within 0.1 cm³

    • You should normally carry out at least three titrations

    • Record all of the results that you obtain.

13. Calculate / record the mean (average) volume of sodium hydrogensulfate solution used in the

    titration

    • Show your working

14. Use your results to calculate the concentration of the sodium hydroxide

    • Show your working

Diagram

Practical Tip

• Completing a rough titration first helps to determine the approximate end-point of the titration

• This also allows you to run the burette until 2 - 3 cm³ before the end-point and slowly add the sodium hydrogensulfate solution dropwise

Results

• Record your results for each test carefully in a suitable table like the one below: 

Evaluation

• Identify the concordant results

  • Remember: Concordant results are within 0.1 cm³

• Calculate the mean average titre

• Calculate the moles of the chemical with the known concentration

  • In this case, this is the sodium hydrogensulfate

• Use the balanced chemical equation to deduce the moles of the chemical with the unknown concentration

  • In this case, this is the sodium hydroxide

• Calculate the concentration of the chemical with the unknown concentration

  • Careful: This may require the volume converting from cm³ to dm³