STOIKIOMETRI (PART 7) MENENTUKAN RUMUS AIR KRISTAL / GARAM HIDRAT

Introduction

This tutorial will guide you through the process of determining the formula of hydrated salts (air kristal) by removing water molecules from their crystalline structure. By following these steps, you will gain a clear understanding of how to calculate the number of water molecules associated with a salt, an important concept in stoichiometry.

Step 1: Understand Hydrated Salts

• Hydrated salts are solid compounds that contain water molecules as part of their crystal structure.
• The presence of water affects the physical properties of the salt, such as its color and weight.
• Common examples include copper(II) sulfate pentahydrate (CuSO₄·5H₂O) and magnesium sulfate heptahydrate (MgSO₄·7H₂O).

Step 2: Heating the Hydrated Salt

• To determine the formula, you need to release the water of hydration.
• Procedure:
  1. Weigh the hydrated salt accurately using a balance.
  2. Place the salt in a clean, dry crucible.
  3. Heat the crucible gently using a Bunsen burner or hot plate.
  4. Continue heating until you notice a change in the appearance (usually a color change) and the mass stabilizes.
• Tip: Avoid overheating to prevent decomposition of the salt.

Step 3: Calculate the Mass Loss

• After heating, allow the crucible to cool and weigh it again.
• Calculate the mass of water lost:
  • Mass of hydrated salt before heating - Mass of anhydrous salt after heating = Mass of water lost.
• Common Pitfall: Ensure that the crucible is completely dry before weighing to avoid inaccurate results.

Step 4: Determine the Molar Masses

• Calculate the molar mass of the anhydrous salt and the water:
  • Anhydrous salt: Use the periodic table to sum the atomic masses of its constituent elements.
  • Water (H₂O): Molar mass = 18.02 g/mol.

Step 5: Calculate the Number of Moles

• Use the formula:
  • Number of moles = Mass (g) / Molar mass (g/mol).
• Example Calculation:
  • If you lost 18 grams of water, then:
    • Moles of water = 18 g / 18.02 g/mol ≈ 1 mole.

Step 6: Establish the Hydration Ratio

• Calculate the number of moles of the anhydrous salt using the same method.
• Establish the ratio of moles of water to moles of the anhydrous salt.
• Final Formula: Write the formula of the hydrated salt in the form of Salt·nH₂O, where n is the mole ratio.

Conclusion

Determining the formula for hydrated salts involves heating the salt to release water, measuring the mass loss, calculating the number of moles of both the anhydrous salt and the water, and establishing their ratio. Understanding this process is crucial for mastering stoichiometry in chemistry. For further practice, consider reviewing related stoichiometry concepts or attempting additional exercises on hydrated salts.