How do you calculate water of hydration?

Calculating the water of hydration, also known as water of crystallization, involves determining the percentage of water present in a hydrated compound. Here's how you do it:

Calculating Water of Hydration Percentage

The percentage of water in a hydrated compound is calculated by:

Divide the mass of the water lost by the mass of hydrate and multiply by 100. The theoretical (actual) percent hydration (percent water) can be calculated from the formula of the hydrate by dividing the mass of water in one mole of the hydrate by the molar mass of the hydrate and multiplying by 100.

Mathematically, this can be represented as:

% Water of Hydration = (Mass of Water / Mass of Hydrated Compound) * 100

Steps for Calculating Water of Hydration (Experimental Data)

Here's a step-by-step guide, typically used in a lab setting, when you have experimental data:

1. Determine the mass of the hydrated compound. This is your starting material.

2. Heat the hydrated compound to drive off the water.

3. Determine the mass of the anhydrous compound (the compound without water) after heating. Make sure all the water has been driven off.

4. Calculate the mass of water lost: Subtract the mass of the anhydrous compound from the mass of the hydrated compound.

   Mass of Water = Mass of Hydrated Compound - Mass of Anhydrous Compound

5. Calculate the percentage of water of hydration:

   % Water of Hydration = (Mass of Water / Mass of Hydrated Compound) * 100

Calculating Water of Hydration (Theoretical Value)

If you know the chemical formula of the hydrate, you can calculate the theoretical percentage of water.

1. Determine the molar mass of water (H₂O): Approximately 18.015 g/mol.

2. Determine the number of water molecules in one formula unit of the hydrated compound (e.g., CuSO₄·5H₂O has 5 water molecules).

3. Calculate the total mass of water in one mole of the hydrate: Multiply the molar mass of water by the number of water molecules.

4. Calculate the molar mass of the entire hydrated compound.

5. Calculate the percentage of water of hydration:

   % Water of Hydration = (Mass of Water in One Mole of Hydrate / Molar Mass of Hydrate) * 100

Example

Let's say you heated 5.00 g of a hydrated salt (CuSO₄·xH₂O) and obtained 3.20 g of the anhydrous salt (CuSO₄).

1. Mass of Hydrated Compound: 5.00 g
2. Mass of Anhydrous Compound: 3.20 g
3. Mass of Water Lost: 5.00 g - 3.20 g = 1.80 g
4. % Water of Hydration: (1.80 g / 5.00 g) * 100 = 36.0%

Therefore, the hydrated salt contains 36.0% water by mass.