Percent of Water in an Unknown Hydrated Salt

Objectives

• To determine the percent (by mass) of water in an unknown hydrated salt.

• To determine the number of water molecules per formula unit of the unknown hydrated salt.

Hydrates: Definition and Properties

Many ionic compounds can form crystalline solids that contain water molecules as part of their structure. These compounds are known as hydrates. The water molecules are incorporated into the crystal lattice and are called water of hydration. Hydrates have a definite composition, with a specific number of water molecules associated with each formula unit of the salt.

• Hydrated salt: An ionic compound containing water molecules in its crystal structure.

• Anhydrous salt: The compound after water of hydration has been removed, typically by heating.

• Water of hydration: Water molecules chemically bound within the crystal lattice of a salt.

Examples of common hydrates:

Stability and Loss of Water in Hydrates

Some hydrates lose water spontaneously when exposed to air, a process called efflorescence. Others require heating to drive off the water of hydration. The process of removing water from a hydrate is reversible; anhydrous salts can absorb water to reform hydrates.

Experimental Determination of Water Content

To determine the percent of water in a hydrate, the sample is weighed, heated to remove water, and weighed again. The difference in mass corresponds to the water lost.

• Equation for percent water:

• Equation for moles of water per formula unit:

Experimental Procedure Overview

• Prepare and weigh a clean, dry crucible.

• Add the unknown hydrate and weigh again.

• Heat the crucible to drive off water, then cool and reweigh.

• Repeat heating and weighing until mass is constant (indicating all water has been removed).

• Calculate mass of water lost and percent water using the equations above.

• Determine moles of water and moles of anhydrous salt to find the number of water molecules per formula unit.

Sample Data and Calculations

• Percent water calculation:

• Moles of water:

• Moles of anhydrous salt:

• Number of water molecules per formula unit: (rounded to nearest whole number: 6)

Formula for hydrated salt: CuSO4·6H2O

Additional info: The actual hydrate for copper(II) sulfate is typically pentahydrate (CuSO4·5H2O), but experimental error or sample variation may result in a different calculated value.

Reproducibility and Accuracy

• Repeat measurements to ensure results are consistent.

• If two trials agree within 0.05 g, the result is considered reproducible.

• Sources of error include incomplete drying, sample loss, or inaccurate weighing.

Summary Table: Key Terms and Definitions

Example Application

Determining the percent water in a hydrate is a common laboratory exercise in introductory chemistry courses. It illustrates the use of mass measurements, stoichiometry, and the concept of chemical formulas for hydrated compounds.