A Level Chemistry Revision "Determining Water of Crystallisation by Titration Part 2"

Introduction

This tutorial guides you through the process of determining the water of crystallisation in a compound using titration. Understanding this method is essential for A Level Chemistry students, particularly those adhering to the UK curriculum. By following these steps, you will improve your calculation skills and avoid common pitfalls.

Step 1: Prepare Your Materials

Gather the necessary materials for your titration experiment. This includes:

• A sample of the compound containing water of crystallisation
• A standard solution for titration (e.g., sodium hydroxide)
• A burette
• A pipette and pipette filler
• A conical flask
• A white tile (to help observe the color change)
• Indicator solution (e.g., phenolphthalein)

Practical Advice

• Ensure all glassware is clean to avoid contamination.
• Familiarize yourself with the titration setup before starting the experiment.

Step 2: Perform the Titration

Carry out the titration carefully to obtain accurate results.

1. Measure the Sample: Use the pipette to transfer a specific volume of the sample solution into the conical flask.
2. Add Indicator: Add a few drops of the chosen indicator to the sample in the flask.
3. Fill the Burette: Fill the burette with the standard solution, ensuring there are no air bubbles in the nozzle.
4. Titrate: Gradually add the standard solution from the burette to the sample, swirling the flask continuously until you reach the endpoint (indicated by a color change).
5. Record the Volume: Note the volume of the titrant used to reach the endpoint.

Practical Advice

• Add the titrant slowly as you approach the endpoint to avoid overshooting.
• Perform the titration at least three times to ensure consistency in results.

Step 3: Calculate Water of Crystallisation

After completing the titration, it's time to calculate the water of crystallisation.

1. Calculate Moles of Titrant:

   • Use the formula:
     [ \text{Moles} = \text{Concentration} \times \text{Volume (in dm}^3\text{)} ]
   • Convert the volume from mL to dm³ by dividing by 1000.

2. Determine the Moles of the Sample:

   • Use the stoichiometry from the balanced chemical equation to find the moles of the sample that reacted.

3. Calculate Water of Crystallisation:

   • Express the moles of water associated with the compound. This is often represented as a ratio.
   • Use the formula:
     [ \text{Water of Crystallisation} = \frac{\text{Moles of Water}}{\text{Moles of Anhydrous Compound}} ]

Common Pitfalls

• Forgetting to account for the dilution of the sample can lead to incorrect calculations.
• Ensure the balanced equation used is correct for accurate stoichiometry.

Conclusion

Determining the water of crystallisation through titration involves careful preparation, execution, and calculation. By following the steps outlined above, you can accurately assess the water content in your samples. Remember to practice the titration technique multiple times to gain confidence and accuracy in your results. For further study, review the balanced equations relevant to your compound and practice similar calculations to reinforce your understanding.