Short Answer
Water of crystallization is determined by heating a known mass of a hydrated salt to remove the water present in its crystal structure. The difference in mass before and after heating shows how much water was lost. Using this mass loss, the number of water molecules attached to each formula unit of the salt can be calculated.
This method helps chemists find the exact number of water molecules in hydrated compounds. Such information is important for writing correct formulas, studying chemical behaviour, and identifying substances in laboratories.
Detailed Explanation
Determination of Water of Crystallization
Determining water of crystallization means finding out how many water molecules are chemically attached to each formula unit of a hydrated salt. These water molecules are part of the crystal structure, not free water. To determine them, the salt is heated to remove the water, and the loss in mass is measured. This process reveals the number of water molecules present in the crystal.
This method is commonly used in school laboratories, industries, and research to analyse salts such as copper sulphate, magnesium sulphate, and sodium carbonate.
Step 1: Weigh the hydrated salt
The first step is to take a known mass of the hydrated salt. A clean, dry crucible is used, and the mass of the salt along with the crucible is recorded. This value includes both the salt and its water of crystallization.
For example, if the mass of hydrated copper sulphate (CuSO₄·5H₂O) and the crucible together is 25 g, and the crucible alone weighs 20 g, then the mass of the hydrated salt is:
25 g − 20 g = 5 g
Step 2: Heat the hydrated salt
The hydrated salt is then heated strongly. As heating continues, the water of crystallization evaporates from the salt. The salt may show visible changes such as:
- Colour change
- Becoming powdery
- Loss of shine
For example, blue CuSO₄·5H₂O becomes white CuSO₄.
Step 3: Weigh the anhydrous salt
After heating, the salt is allowed to cool in a desiccator to prevent absorption of moisture from the air. Then it is weighed again. The mass now represents only the anhydrous salt (salt without water).
For example, after heating, suppose the mass becomes 23.1 g. The mass of anhydrous salt is:
23.1 g − 20 g = 3.1 g
Step 4: Calculate the mass of water lost
The difference in mass before and after heating gives the mass of the water removed:
Mass of water lost = Mass of hydrated salt − Mass of anhydrous salt
Using the earlier values:
5 g (hydrated sample) − 3.1 g (anhydrous sample) = 1.9 g of water
Step 5: Convert masses to moles
To find the number of water molecules, the masses of both the anhydrous salt and the lost water are converted into moles:
Moles of water = Mass of water ÷ 18
Moles of anhydrous salt = Mass of salt ÷ Molar mass of salt
These mole values show how many moles of water correspond to one mole of the salt.
Step 6: Find the mole ratio
The ratio between moles of water and moles of anhydrous salt gives the number of water molecules per formula unit. This becomes the “x” in the formula:
Salt · xH₂O
For many salts, this ratio is a simple whole number, such as 5, 7, or 10.
Example for clarity
Suppose the moles are:
- Moles of salt = 0.02
- Moles of water = 0.10
Mole ratio = 0.10 ÷ 0.02 = 5
So, the formula of the hydrated salt would be:
Salt · 5H₂O
Importance of determining water of crystallization
Determining water of crystallization is important because:
- It helps identify unknown salts
- It ensures correct chemical formulas
- It explains colour and physical properties of crystals
- It shows how heating affects chemical compounds
- It helps in preparing chemicals with correct hydration levels
For example, plaster of Paris is made from gypsum by removing part of its water of crystallization. Detergent industries also use this information to maintain correct hydration levels.
Factors affecting accuracy
The accuracy of water of crystallization determination depends on:
- Proper heating to remove all water
- Preventing reabsorption of moisture
- Correct measurement of masses
- Using pure samples
Careful laboratory technique ensures correct results.
Role in real-life chemistry
Many chemicals used in medicine, industries, and laboratories contain water of crystallization. Determining this water helps maintain product quality and ensures correct behaviour during use. Hydrated salts are also used in temperature regulation, food processing, and construction materials.
Conclusion
Water of crystallization is determined by heating a hydrated salt to remove its water and measuring the mass difference. By converting the lost water and remaining salt into moles, chemists calculate the number of water molecules attached to each formula unit. This process helps identify hydrated compounds, understand their properties, and use them accurately in laboratories and industries.