Determine the water of crystallization and empirical stoichiometric formula of hydrated copper(II) sulfate (CuSO₄·xH₂O) through thermal desiccation in a porcelain crucible, monitor stepwise phase transitions to anhydrous CuSO₄, and extract the stoichiometric hydration number x via ordinary least-squares linear regression. Explore interactive analytical gravimetry, Class A volumetric glassware and density determination.
Ready to dispense 4.5 g of liquid.
Adjust hydrate mass V, then weigh the dispensed aliquot.
Mass observations are determined by the high-precision thermodynamic equation of state for pure liquid water. Readings reflect thermal equilibrium and analytical balance force restoration.
The gravimetric determination of hydrate stoichiometry assumes complete dehydration to pure anhydrous salt without secondary thermal decomposition. In practice, minor experimental limitations include: (1) Trace mechanical decrepitation where micro-particulate salt crystals burst and escape the crucible; (2) Residual lattice water retention if calcination temperatures or durations are insufficient; (3) Atmospheric re-hydration during the brief transfer of the cooled crucible from the desiccator to the analytical balance pan; and (4) Trace chemical impurities in analytical reagent grade salts (such as iron or nickel sulfates) which exhibit slightly different hydration stoichiometries. Thermal gravimetric analysis (TGA) of pure crystalline copper(II) sulfate pentahydrate reveals that dehydration occurs through three distinct stoichiometric steps rather than a single continuous loss: (1) loss of two coordinated water molecules between 63 °C and 90 °C to form trihydrate CuSO₄·3H₂O; (2) loss of two additional water molecules between 105 °C and 125 °C to form monohydrate CuSO₄·H₂O; and (3) loss of the final structural hydrogen-bonded water molecule between 210 °C and 250 °C to yield anhydrous CuSO₄. Overheating above 600 °C causes irreversible decomposition of anhydrous CuSO₄ into black copper(II) oxide (CuO) and toxic sulfur trioxide gas (SO₃). The molar masses are M(CuSO₄) = 159.609 g/mol and M(H₂O) = 18.0154 g/mol. Under ordinary least-squares regression of water mass lost m_water against recovered anhydrous salt mass m_anhydrous, the theoretical slope is m_slope = (5 · 18.0154) / 159.609 = 0.564360 g water per g salt, yielding exactly x = 5.0000 moles of H₂O per mole of CuSO₄.
Withstands severe thermal shock up to 800 °C over an open flame without cracking, providing an inert, non-reactive vessel for calcining hydrated inorganic salts and containing crystal samples during thermal desiccation.
Supports the conical base of the porcelain crucible securely above the gas burner flame, with refractory ceramic clay sleeves insulating the nichrome wires and distributing thermal energy evenly across the crucible walls.
Provides fine micro-metering of fuel gas and air intake to establish an oxidizing, non-luminous blue flame for gentle initial drying and subsequent strong calcination without sooty carbon deposition.
Cools hot crucibles in an ultra-dry atmosphere (<0.005 mg/L H₂O vapor) over cobalt-chloride-impregnated anhydrous calcium sulfate, preventing rapid hygroscopic reabsorption of atmospheric moisture by the anhydrous salt.
Permits safe, contamination-free transfer of heated crucibles between the ring stand, wire gauze, desiccator, and analytical balance without transferring finger grease or introducing thermal shock.
Measures crucible tare mass, initial hydrated sample mass, and successive dehydrated residues to four decimal places (±0.0001 g) inside an enclosed draft shield chamber.
Determine the water of crystallization and empirical stoichiometric formula of hydrated copper(II) sulfate (CuSO₄·xH₂O) through thermal desiccation in a porcelain crucible, monitor stepwise phase transitions to anhydrous CuSO₄, and extract the stoichiometric hydration number x via ordinary least-squares linear regression. Suggested independent volumetric settings: 1.5, 2.5, 3.5, 4.5, 5.5, 6.5, 7.5 g.
Clean a high-form porcelain crucible and lid with warm 3 M nitric acid, rinse thoroughly with deionized water, and mount on the clay triangle. Ignite the Tirrill burner and heat the crucible to dull red heat (~600 °C) for 10 minutes to burn off volatile residues. Transfer using tongs to the desiccator, cool for 15 minutes to room temperature, and weigh to ±0.0001 g. Reheat for 5 minutes, cool, and reweigh until successive tare readings agree within ±0.0002 g.
Add approximately the designated sample mass (1.5 to 7.5 g) of finely crushed, crystalline azure-blue copper(II) sulfate hydrate into the pre-calcined crucible. Record the exact combined mass of crucible, lid, and hydrated salt on the analytical balance. Calculate the exact initial mass of hydrated salt m_hydrate by subtracting the constant crucible tare mass.
Place the crucible containing the blue hydrate onto the clay triangle with the lid placed slightly ajar (leaving a 2–3 mm crescent opening) to permit evolving water vapor to escape freely while preventing violent crystal spattering. Adjust the burner flame to a low, gentle brush flame, heating slowly for 5 to 7 minutes until the initial blue crystals lose their crystalline luster and crumble into a pale turquoise-cyan powder.
Gradually increase the burner flame intensity until the base of the crucible glows faint dull red (~250 °C to 300 °C). Maintain this calcination temperature for 12 to 15 minutes, observing through the ajar lid until all traces of blue or green coloration completely vanish, leaving a uniform, chalky grayish-white anhydrous copper(II) sulfate powder. Ensure temperature does not exceed 500 °C to prevent decomposition of sulfate to black copper(II) oxide.
Using crucible tongs, immediately seat the lid completely flush to seal the crucible. Transfer the hot assembly to the desiccator, slide the greased desiccator lid closed, and allow the system to cool for 20 minutes to ambient room temperature. Weigh the closed crucible immediately on the analytical balance. Reheat on the clay triangle for 5 minutes, cool in the desiccator, and reweigh to confirm that constant mass has been achieved within ±0.0003 g.
Calculate the mass of water vaporized and evolved: m_water = m_hydrate - m_anhydrous. Compute the recovered anhydrous salt mass: m_anhydrous = m_final - m_tare. Repeat across the sample series from 1.5 g to 7.5 g. Construct an ordinary least-squares regression plot of m_water against m_anhydrous. Extract the stoichiometric hydration multiplier x = m_slope · (M_salt / M_water) and compare with the theoretical pentahydrate value x = 5.0000.
Points are your recorded gravimetric observations. The regression line validates the linear mass-volume proportionality and extracts density.
| # | Initial hydrated salt mass m_hydrate (g) (g) | Mass of water evolved m_water (g) | Anhydrous salt mass m_anhydrous (g) | Mass of water lost m_water (g) | Remove |
|---|
No readings yet. Dispense and weigh a volumetric aliquot to record it.