Percent of Water in an Unknown Hydrated Salt: Experimental Analysis and Calculations

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:

Hydrated Salt	Formula
Copper(II) sulfate pentahydrate	CuSO4·5H2O
Barium chloride dihydrate	BaCl2·2H2O
Sodium carbonate decahydrate	Na2CO3·10H2O

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:

$\text{\% water} = \frac{\text{mass of water lost}}{\text{mass of hydrate}} \times 100$

• Equation for moles of water per formula unit:

$\text{Number of water molecules per formula unit} = \frac{\text{moles of water lost}}{\text{moles of anhydrous salt}}$

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

Measurement	Trial 1	Trial 2
Mass of empty crucible	12.95 g
Mass of crucible + hydrate	27.16 g
Mass of hydrate	14.21 g
Mass of crucible + anhydrous salt (after heating)	26.07 g
Mass of anhydrous salt	8.32 g
Mass of water lost	5.89 g

• Percent water calculation:

$\text{\% water} = \frac{5.89\,\text{g}}{14.21\,\text{g}} \times 100 = 41.45\%$

• Moles of water: $\frac{5.89\,\text{g}}{18.02\,\text{g/mol}} = 0.327\,\text{mol}$

• Moles of anhydrous salt: $\frac{8.32\,\text{g}}{159.61\,\text{g/mol}} = 0.0521\,\text{mol}$

• Number of water molecules per formula unit: $\frac{0.327}{0.0521} \approx 6.28$ (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

Term	Definition
Hydrate	An ionic compound containing water molecules in its crystal structure.
Anhydrous salt	The salt after water of hydration has been removed.
Water of hydration	Water molecules bound within the crystal lattice of a salt.
Efflorescence	Loss of water from a hydrate when exposed to air.

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.