Core Practical 2.43C

Core Practical 2.43C: Preparation of Lead(II) Sulfate

Revision guide containing method, variables, safety, sample calculations, and model exam answers.

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Scientific Principles & Theory

Scientific Background: An insoluble salt can be prepared by a precipitation reaction. This involves mixing solutions of two soluble salts. The ions in solution swap partners, and the positive and negative ions of the insoluble salt bind strongly to form a solid precipitate:

Pb(NO₃)₂(aq) + Na₂SO₄(aq) → PbSO₄(s) + 2NaNO₃(aq)

Because the product is a solid precipitate, it can be easily separated from the remaining soluble salts by filtration, washed with distilled water to remove impurities, and then dried.

Experimental Variables

Independent Variable

The concentrations of the starting soluble salt solutions.

Dependent Variable

The mass and purity of the dry lead(II) sulfate precipitate.

Control Variables

Volumes of solutions mixed (to ensure stoichiometry), wash water temperature and volume.

⚠️ Lab Risk Assessment

Hazard Associated Risk Control Measure
Lead(II) compounds (toxic) Lead poisoning (neurological and organ damage) Wear safety goggles and chemical-resistant gloves; wash hands immediately; clean up spills with wet paper towels.
Soluble nitrate wastes Environmental toxicity Dispose of all lead-containing wastes in a designated heavy metal waste container, never down the sink.

Apparatus & Procedure

Required Apparatus

  • Beakers (two 100 cm³)
  • Filter funnel and filter paper
  • Conical flask
  • Glass stirring rod
  • Wash bottle with distilled water
  • Watch glass or evaporating basin (for drying)
  • Oven (set to ~50 °C)
  • Digital balance (0.01 g resolution)
  • 0.1 mol/dm³ lead(II) nitrate solution
  • 0.1 mol/dm³ sodium sulfate solution

Step-by-Step Procedure

  1. Measure equal volumes (e.g. 25 cm³) of 0.1 mol/dm³ lead(II) nitrate and 0.1 mol/dm³ sodium sulfate using measuring cylinders.
  2. Pour the lead(II) nitrate solution into a clean beaker, then add the sodium sulfate solution.
  3. Stir the mixture thoroughly with a glass rod. A dense white precipitate of lead(II) sulfate will form immediately.
  4. Set up a filter funnel with filter paper over a conical flask.
  5. Pour the mixture into the filter funnel. The lead(II) sulfate precipitate will collect on the filter paper (residue), while the sodium nitrate solution passes through into the flask (filtrate).
  6. Rinse the beaker with a small volume of distilled water and pour it through the filter to ensure all solid is transferred.
  7. Wash the precipitate on the filter paper with several small portions of distilled water. This removes any remaining soluble sodium nitrate solution clinging to the lead(II) sulfate.
  8. Carefully remove the filter paper containing the wet lead(II) sulfate residue from the funnel.
  9. Transfer the precipitate onto a watch glass or evaporating basin.
  10. Dry the solid in a warm oven (at approximately 50 °C) or leave it in a warm room to evaporate all water. Weigh the dry product.
Precipitation & Washing Process Stirring precipitate Distilled wash bottle

Fig 1. Laboratory experimental setup for Core Practical 2.43C.

Sample Data & Calculations

This representative dataset illustrates the values typically obtained when carrying out this experiment in the laboratory:

Parameter Measured Mass Value (g)
Mass of empty watch glass 18.90
Mass of watch glass + wet precipitate 22.45
Mass of watch glass + dry precipitate (after drying) 21.82

Data Processing & Analysis

  1. Mass of dry lead(II) sulfate precipitate = 21.82 g - 18.90 g = 2.92 g
  2. Equation: Pb(NO₃)₂ + Na₂SO₄ → PbSO₄ + 2NaNO₃
  3. If we used 0.010 mol of both reactants, theoretical yield of PbSO₄ = 0.010 mol * 303.3 g/mol = 3.03 g
  4. Percentage Yield = (2.92 g / 3.03 g) * 100 = 96.4%

Conclusion & Evaluation

Chemical Explanation: Saturated solutions are heavily dependent on temperature. Heating shifts solubility limits, allowing more solute to form coordinate bonds or ion-dipole interactions with solvent molecules. When cooled, the reverse process happens and solute precipitates out.

Experimental Error Analysis

Error Type & Source Effect on Final Result Mitigation Strategy
Systematic Error
Incomplete washing of precipitate		 Soluble sodium nitrate salts remain on the filter paper and dry along with the lead sulfate, making the recorded mass too high. Wash the precipitate thoroughly with multiple portions of distilled water before removing it from the funnel.
Systematic Error
Precipitate not completely dry when weighed		 Remaining water increases the mass, giving an artificially high yield. Dry in an oven and check constant mass: dry, weigh, dry again, weigh, until mass is constant.

Exam Practice

Exam-Style Design Question (6 Marks)

Describe how you would prepare a pure, dry sample of lead(II) sulfate crystals starting from solid lead(II) nitrate and solid sodium sulfate. Detail the preparation of the solutions as well as the isolation steps.

View Model Answer & Mark Scheme

Model Answer (6/6 Marks):

  1. Dissolving: Dissolve separate, measured quantities of solid lead(II) nitrate and solid sodium sulfate in distilled water in two separate beakers to make aqueous solutions.
  2. Mixing: Pour the two solutions together into a single beaker and stir with a glass rod. A white precipitate of lead(II) sulfate will form.
  3. Filtration: Pour the mixture through a filter funnel lined with filter paper. The insoluble lead(II) sulfate remains on the paper as the residue.
  4. Washing: Wash the residue on the filter paper with distilled water to rinse away any soluble sodium nitrate solution.
  5. Drying: Transfer the residue to a watch glass and place in a warm oven or leave to dry. Weigh the product to ensure constant mass.
Examiner Tip:

You must explain how to prepare the aqueous solutions first if starting from solid salts (dissolving them in distilled water in separate beakers).