Stoichiometry of Hydrates: Calculations and Examples

📚 Quick Study Guide

• 🧊 Hydrates: Compounds that have a specific number of water molecules incorporated into their crystal structure. For example, $CuSO_4 \cdot 5H_2O$ is copper(II) sulfate pentahydrate.
• ⚖️ Formula Mass: The formula mass of a hydrate includes the mass of the anhydrous salt plus the mass of the water molecules.
• 🔥 Anhydrous Salt: The compound remaining after the water of hydration has been removed (usually by heating).
• 💧 Water of Hydration: The water molecules associated with each formula unit of the salt.
• ⚗️ Stoichiometry: The quantitative relationship between reactants and products in a chemical reaction (or in this case, the ratio of salt to water in a hydrate).
• 📐 Calculations:
  • Determine the mass of water lost when heating a hydrate.
  • Convert the mass of anhydrous salt and water to moles.
  • Find the mole ratio of water to anhydrous salt.
  • Use the mole ratio to determine the formula of the hydrate.

🧪 Practice Quiz

1. What is a hydrate?
   1. A compound containing only hydrogen and oxygen.
   2. A compound that readily dissolves in water.
   3. A compound with water molecules incorporated into its crystal structure.
   4. A compound that repels water.
2. When a hydrate is heated, what is typically released?
   1. Oxygen gas
   2. Hydrogen gas
   3. Water vapor
   4. Carbon dioxide
3. What is the term for the compound remaining after the water of hydration has been removed?
   1. Hydrated salt
   2. Anhydrous salt
   3. Aqueous solution
   4. Saturated solution
4. A hydrate of $CuSO_4$ is heated, and it loses 36.08 g of water. If the mass of anhydrous $CuSO_4$ remaining is 63.92 g, what is the mole ratio of water to $CuSO_4$?
   1. 2:1
   2. 3:1
   3. 4:1
   4. 5:1
5. What does the “5” represent in the formula $NiCl_2 \cdot 6H_2O$?
   1. The charge of nickel
   2. The number of nickel atoms
   3. The number of water molecules per formula unit of $NiCl_2$
   4. The oxidation state of chlorine
6. If you have 2.0 moles of $BaCl_2 \cdot 2H_2O$, how many moles of water do you have?
   1. 1.0 mole
   2. 2.0 moles
   3. 3.0 moles
   4. 4.0 moles
7. Which of the following is NOT a step in determining the formula of a hydrate?
   1. Determine the mass of water lost.
   2. Convert masses to moles.
   3. Find the mole ratio of water to anhydrous salt.
   4. Measure the pH of the solution.