A Level Chemistry Revision "Water of Crystallisation Part 2"

Introduction

This tutorial provides a step-by-step guide on determining the water of crystallisation in a compound using the heating method. This process is essential for A Level Chemistry students as it helps understand the composition of hydrated compounds. By the end of this guide, you'll have a clear understanding of the procedure and be able to analyze results effectively.

Step 1: Prepare Your Materials

Before starting the experiment, gather the necessary materials:

• A hydrated salt sample (e.g., copper(II) sulfate pentahydrate)
• A crucible and lid
• A balance for measuring mass
• A Bunsen burner or hot plate
• A heat-resistant mat
• Tongs or heat-proof gloves
• A desiccator (optional, for cooling the crucible)

Practical Advice

• Ensure that the crucible is clean and dry before use.
• Accurately measure the mass of the empty crucible to get a baseline for your results.

Step 2: Measure the Mass of the Hydrated Salt

1. Weigh the hydrated salt and record the mass.
2. Add the hydrated salt to the crucible.
3. Measure and record the total mass of the crucible with the salt.

Example Calculation

If the empty crucible weighs 20 g and the crucible with the hydrated salt weighs 25 g, the mass of the hydrated salt is:

• 25 g - 20 g = 5 g

Step 3: Heat the Sample

1. Place the crucible on a heat-resistant mat.
2. Use tongs to position the crucible over the flame of the Bunsen burner.
3. Heat gently at first to avoid splattering, then increase the heat once the water starts to evaporate.

Tips for Effective Heating

• Keep the lid slightly ajar to allow steam to escape.
• Monitor the color change in the salt to identify when it has fully dehydrated.

Step 4: Cool and Reweigh the Crucible

1. After heating for several minutes, remove the crucible from the heat using tongs.
2. Allow it to cool, preferably in a desiccator to prevent moisture absorption.
3. Once cooled, weigh the crucible with the anhydrous salt and record this mass.

Important Notes

• Ensure the crucible is completely cool before weighing to avoid inaccurate mass readings.

Step 5: Analyze Your Results

1. Calculate the mass of water lost by subtracting the mass of the anhydrous salt from the mass of the hydrated salt.
2. Use the following formula to determine the moles of the hydrated and anhydrous salt:
   • Moles = Mass (g) / Molar Mass (g/mol)

Example Calculation

If the mass of the hydrated salt was 5 g and the mass of the anhydrous salt is 3 g:

• Mass of water lost = 5 g - 3 g = 2 g
• Moles of water = 2 g / 18 g/mol (molar mass of water) = 0.111 moles

Step 6: Review Potential Problems

• Incomplete dehydration, leading to inaccurate results.
• Loss of salt during heating.
• Absorption of moisture during cooling.

Common Pitfalls to Avoid

• Do not overheat the salt, as this can lead to decomposition.
• Ensure that measurements are as precise as possible to improve accuracy.

Conclusion

In this guide, we covered how to determine the water of crystallisation using the heating method. Remember to carefully measure, heat, and analyze your results to achieve accurate findings. Practice with different hydrated salts to enhance your understanding and application of these concepts in your A Level Chemistry studies.