Percent of Water in an Unknown Hydrated Salt

Objectives

• Determine the percent by mass of water in an unknown hydrated salt.
• Calculate the number of water molecules ( n ) associated with each formula unit of the salt (i.e., find the value of n in “salt– n $H_2O$”).

Key Vocabulary & Concepts

• Hydrate: Ionic crystal that contains “water of hydration.”
• Anhydrous salt: Compound that remains after all water has been driven off.
• Water of hydration: Water molecules chemically bound in the crystal lattice.
• Efflorescent compound: Spontaneously loses water to the atmosphere.
• Hygroscopic substance: Absorbs moisture from the air (e.g., $CuSO_4$).
• Deliquescent substance: Absorbs so much moisture that it dissolves in the absorbed water (e.g., $CaCl_2$).
• Heating to constant mass: Repeated heating/cooling cycles until two successive masses differ by (<0.05\,\text{g}). Ensures complete removal of water.

Representative Hydrate Formulas & Names

• $CuSO<em>4{\cdot}5H</em>2O$ → copper(II) sulfate pentahydrate
• $BaCl<em>2{\cdot}2H</em>2O$ → barium chloride dihydrate
• $Na<em>2CO</em>3{\cdot}10H_2O$ → sodium carbonate decahydrate

Chemical Principles & Reversibility

• General dehydration reaction:
  $\text{hydrated salt (s)} \xrightarrow{\text{heat}} \text{anhydrous salt (s)} + \text{water (g)}$
• Example: $CuSO<em>4{\cdot}5H</em>2O(\text{blue}) \xrightarrow{\text{heat}} CuSO<em>4(\text{white}) + 5H</em>2O(g)$
• Reverse process occurs on exposure to moisture: anhydrous salt re-forms the hydrate.

Percent-Composition Equation (Experiment Focus)

• Percent water:
  $\%\,\text{H}_2\text{O} = \frac{\text{mass of water lost}}{\text{mass of hydrate}} \times 100$
• Mole ratio for hydrate formula:
  $n = \frac{\text{moles of water}}{\text{moles of anhydrous salt}}$
  (Round n to the nearest whole number.)

Experimental Procedure

A. Crucible Preparation

• Heat clean crucible on clay triangle with blue flame for “several minutes.”
• Cool on ceramic tile (≥3\,\text{min}); never place hot crucible on balance or benchtop.
• From this point onward, handle crucible only with tongs.
• Weigh cooled, empty crucible; record (e.g., $0.0X\,\text{g}$).

B. Initial Heating of Hydrate

1. Obtain unknown hydrate; note its ID number.
2. Add unknown until crucible is ~1/3 full; weigh crucible + hydrate.
3. Gentle heat → then stronger heat for 10 min.
   • If sample liquefies: maintain low heat until resolidified, then increase heat.
   • If not: steadily ramp to strong heat.
4. Cool and weigh crucible + partially dehydrated contents.

C. Heating to Constant Mass

1. Reheat 5 min, cool, re-weigh.
2. If two consecutive masses differ by (>0.05\,\text{g}), perform a 3rd heating/cooling/weighing cycle.
3. Use the last mass (constant) for calculations; product assumed completely anhydrous.

D. Reproducibility (Trial 2)

• Repeat entire experiment with a fresh portion of unknown to verify results.

Sample Data (Student Nancy Carbajal, Trial 1)

• Mass empty crucible: $15.01\,\text{g}$
• Mass crucible + hydrate: $23.51\,\text{g}$
• Mass of hydrate: $23.51 − 15.01 = 8.50\,\text{g}$
• Mass crucible + anhydrous (constant): $19.65\,\text{g}$
• Mass anhydrous salt: $19.65 − 15.01 = 4.64\,\text{g}$
• Mass water lost: $8.50 − 4.64 = 3.86\,\text{g}$
• Percent water:
  $\%\,\text{H}_2\text{O} = \frac{3.86}{8.50} \times 100 = 45.4\%$ (3 sig figs)

Determining Hydrate Formula (Using Best Trial)

1. Moles of water:
   $n<em>{\text{H}</em>2O}=\frac{3.86\,\text{g}}{18.02\,\text{g\,mol}^{−1}} = 0.214\,\text{mol}$
2. Given molar mass of anhydrous salt (from instructor): $258.2\,\text{g\,mol}^{−1}$
3. Moles of anhydrous salt:
   $n_{\text{salt}} = \frac{4.64\,\text{g}}{258.2\,\text{g\,mol}^{−1}} = 0.0180\,\text{mol}$
4. Mole ratio:
   $n = \frac{0.214}{0.0180} = 11.9 \approx 12$
5. Empirical hydrate formula:
   $\text{salt}{\cdot}12H_2O$ (value rounded to nearest whole number)

Practical & Safety Considerations

• Use a blue, non-luminous flame to prevent soot contamination.
• Do not touch crucible with hands after initial heating (oils → mass error).
• Allow full cooling before weighing; hot air currents create buoyancy, mis-reads balance.
• Record masses to the precision of the balance (typically 0.01 g).

Connections to Foundational Principles

• Mirrors standard percent-composition calculations used in empirical-formula work.
• Heating to constant mass is a classical gravimetric technique (gravimetric analysis).

Real-World Relevance

• Many industrial drying processes (pharmaceuticals, food dehydration) rely on similar gravimetric moisture assays.
• Anhydrous salts (e.g., $CaCl_2$) are commercial desiccants for shipping containers, closets, lab desiccators.

Ethical & Environmental Considerations

• Proper disposal of dehydrated salts prevents environmental contamination; some metal ions (e.g., $Cu^{2+}$) are toxic to aquatic life.
• Accurate moisture assays are crucial where dosing or quality control affects human health (pharmaceutical hydrates).