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

  • CuSO4{\cdot}5H2O → copper(II) sulfate pentahydrate
  • BaCl2{\cdot}2H2O → barium chloride dihydrate
  • Na2CO3{\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: CuSO4{\cdot}5H2O(\text{blue}) \xrightarrow{\text{heat}} CuSO4(\text{white}) + 5H2O(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{\text{H}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).