A Level Chemistry Revision "Water of Crystallisation Part 1"

Introduction

This tutorial provides a comprehensive overview of water of crystallisation, exploring the concepts of hydrated and anhydrous crystals. It aims to equip A Level Chemistry students with the knowledge and skills needed to calculate the amount of water of crystallisation in compounds. Understanding these concepts is crucial for mastering chemical formulas and reactions involving hydrated substances.

Step 1: Understanding Hydrated and Anhydrous Crystals

• Hydrated Crystals: These are crystalline compounds that contain water molecules within their structure. The water molecules are integral to the crystal formation and affect the compound's properties.
• Anhydrous Crystals: These lack water in their structure. When a hydrated crystal is heated, it can lose its water of crystallisation and become an anhydrous compound.

Practical Advice

• Familiarize yourself with common hydrated compounds, such as copper(II) sulfate (CuSO₄·5H₂O), which contains five water molecules.
• Recognize that the loss of water during heating can result in color changes (e.g., blue copper(II) sulfate turns white when anhydrous).

Step 2: Calculating Water of Crystallisation

To calculate the water of crystallisation in a hydrated compound, follow these steps:

1. Identify the Formula: Write down the formula of the hydrated compound.

   • Example: CuSO₄·5H₂O

2. Determine Molar Mass:

   • Calculate the molar mass of both the anhydrous and hydrated forms.

   • For CuSO₄:

     • Cu = 63.55 g/mol
     • S = 32.07 g/mol
     • O (4) = 16.00 g/mol × 4 = 64.00 g/mol
     • Total = 63.55 + 32.07 + 64.00 = 159.62 g/mol

   • For H₂O:

     • H (2) = 1.01 g/mol × 2 = 2.02 g/mol
     • O = 16.00 g/mol
     • Total = 2.02 + 16.00 = 18.02 g/mol

   • For CuSO₄·5H₂O:

     • Total mass = 159.62 + (5 × 18.02) = 159.62 + 90.10 = 249.72 g/mol

3. Calculate the Water of Crystallisation:

   • Use the formula: [ \text{Water of Crystallisation} = \frac{\text{Mass of water in the formula}}{\text{Molar mass of hydrated compound}} \times 100 ]
   • In this example:
     • Water mass = 90.10 g/mol
     • Calculation: [ \text{Percentage of water} = \frac{90.10}{249.72} \times 100 \approx 36.1% ]

Practical Tips

• Keep a periodic table handy for accurate atomic mass calculations.
• Double-check your calculations to avoid errors.

Step 3: Practice Problems

To reinforce your understanding, try these practice questions:

1. Calculate the water of crystallisation for magnesium sulfate heptahydrate (MgSO₄·7H₂O).
2. Determine the percentage of water in barium chloride dihydrate (BaCl₂·2H₂O).

Solutions

• Work through the calculations as shown in Step 2 for each problem.
• Compare your results with a peer or refer to reliable chemistry resources.

Conclusion

Understanding water of crystallisation is essential for A Level Chemistry students, particularly in relation to hydrated and anhydrous compounds. By mastering the calculation methods and practicing problems, you will gain confidence in applying these concepts. For further study, consider exploring more complex compounds and their implications in chemical reactions.