Hydrates and Percent Composition

Introduction to Hydrates

Hydrates are ionic compounds that include water molecules within their crystalline structure. The water is chemically bonded to the salt and is referred to as "water of hydration." The number of water molecules per formula unit is indicated in the compound's formula, such as in nickel(II) sulfate hexahydrate, NiSO4·6H2O.

• Hydrate Formula: Indicates the number of water molecules associated with each formula unit of the salt.

• Example: In NiSO4·6H2O, there are 6 water molecules per formula unit of nickel(II) sulfate.

Calculations Involving Hydrates

Determining Moles and Mass of Water in a Hydrate

To analyze hydrates, it is important to determine the amount of water present in a given amount of the compound, both in moles and grams.

• Moles of Water per Mole of Hydrate: The subscript before H2O in the formula gives the number of moles of water per mole of hydrate.

• Example: In NiSO4·6H2O, there are 6 moles of water per mole of the hydrate.

• Calculating Mass of Water: Multiply the number of moles of water by the molar mass of water ().

• Formula:

• Example: 6 moles × 18.02 g/mol = 108.12 g of water per mole of hydrate.

Calculating Mass of Water in a Given Sample

To find the mass of water in a specific mass of hydrate, use the ratio of the molar mass of water to the molar mass of the hydrate.

• Molar Mass of Hydrate: Add the molar mass of the anhydrous salt and the water of hydration.

• Formula:

• Example Calculation:

  • Ni: 58.69 g/mol

  • S: 32.07 g/mol

  • O4: 4 × 16.00 = 64.00 g/mol

  • 6H2O: 6 × 18.02 = 108.12 g/mol

  • Total: 58.69 + 32.07 + 64.00 + 108.12 = 262.88 g/mol

• Finding Mass of Water in a Sample:

  • Calculate the percent by mass of water in the hydrate.

  • Multiply the percent (as a decimal) by the sample mass.

  • Formula:

Theoretical Percent by Mass of Water in a Hydrate

The percent by mass of water in a hydrate is calculated by dividing the total mass of water in one mole of the hydrate by the molar mass of the hydrate, then multiplying by 100%.

• Formula:

• Example: For NiSO4·6H2O:

  • Mass of water: 108.12 g

  • Molar mass of hydrate: 262.88 g

  • Percent by mass:

Experimental Percent by Mass of Water

In laboratory experiments, the percent by mass of water is determined by measuring the mass of water lost upon heating a known mass of hydrate.

• Formula:

• Example: If 2.500 g of hydrate yields 0.985 g of water:

Percent Error Calculation

Percent error compares the experimental value to the theoretical value to assess accuracy.

• Formula:

• Example: If the theoretical percent is 41.14% and the experimental is 39.4%:

Summary Table: Key Calculations for Hydrates

Applications

• Determining the formula of unknown hydrates by experimental analysis.

• Calculating water content in pharmaceuticals, minerals, and industrial chemicals.