How to Find the Empirical Formula of a Hydrated Salt

A hydrated salt has the general formula: $Salt \cdot xH_2O$. Your goal is to find the value of x (the integer number of water molecules attached to one molecule of the salt). To do this, you must physically burn the water away and measure the weight difference.

1. Weigh the Hydrate: Accurately weigh the blue hydrated salt crystals in a crucible. (e.g., 5.00 grams of hydrated $CuSO_4 \cdot xH_2O$).
2. Heat the Crucible: Heat it strongly over a Bunsen burner. The heat will forcefully evaporate all the trapped water. The blue crystals will turn into a white, dry powder called an Anhydrous Salt.
3. Weigh the Dry Salt: After cooling, weigh the white powder. (e.g., The dry $CuSO_4$ now weighs 3.20 grams).

Step A: Find the mass of the lost water

• Mass of Water = (Mass of Hydrate) - (Mass of Dry Salt)
• Mass of Water = 5.00g - 3.20g = 1.80g of water lost.

Step B: Convert both masses into Moles

• Molar mass of dry $CuSO_4$ = 159.5 g/mol.
  • Moles of dry salt = $3.20 \div 159.5 = 0.020 \text{ moles}$.
• Molar mass of water ($H_2O$) = 18.0 g/mol.
  • Moles of water = $1.80 \div 18.0 = 0.100 \text{ moles}$.

Step C: Find the Ratio (The 'x' value)

• To find the ratio, divide both mole values by the smallest mole value (which is 0.020).
• Salt ratio = $0.020 \div 0.020 = 1$
• Water ratio = $0.100 \div 0.020 = 5$

Conclusion: The value of x is 5. The empirical formula of the hydrated salt is $CuSO_4 \cdot 5H_2O$ (Copper(II) sulfate pentahydrate).