Water of Crystallisation - Calcium Sulfate

• Equation: (\text{CaSO}_4.x\text{H}_2\text{O(s)} \rightarrow \text{CaSO}_4(s) + x\text{H}_2\text{O(g)})
• Method:
  • Weigh an empty crucible and lid
  • Add 2g hydrated calcium sulfate, weigh again
  • Heat strongly, cool, reweigh
  • Repeat heating and weighing until constant mass
• Considerations:
  • Avoid small (0.100g) & large (50g) amounts
  • Use a loose lid to prevent solid loss, allow gas escape
  • Ensure crucible is dry to avoid excessive mass loss

Heating in a Crucible

• Applications: Measure mass change in reactions, e.g., magnesium in oxygen.

Safety and Hazards

• General Safety:
  • Wear goggles for irritants, corrosives
  • Keep flammables away from flames
  • Use gloves for toxics

Practical Guide AQA

• Guide for A-level chemistry practicals and experiments in exams.

Gas Syringe Usage

• Issues:
  • Gas escape, syringe sticking
  • Solubility of gases like CO₂
• Methodology:
  • Measurements depend on room's temperature and pressure

Making a Solution

• Method:
  • Weigh solid, transfer to beaker, dissolve in water
  • Transfer solution to a graduated flask, rinse beaker and funnel
  • Make up to mark with water, invert flask to mix

Diluting a Solution

• Methodology:
  • Pipette 25.0 cm³ into a 250.0 cm³ flask, dilute, invert
  • Volumetric pipette used for accuracy

Titrations

• Equipment:
  • Rinse equipment (pipette, burette, flask)
  • Add indicators (phenolphthalein, methyl orange)
• Procedure:
  • Pipette alkali, touch surface, add acid from burette
  • Use white tile to observe color change
  • Repeat for concordant results

Measuring Enthalpy Change

• Calorimetric Method:
  • Measure temperature change, extrapolate if slow reaction
  • Calculate energy change (Q = m \times c_p \times \Delta T)

Testing for Ions

• Ammonium: Use (\text{NH}_4^+) test with litmus paper
• Group 2 Metals:
  • Magnesium and calcium form hydroxide precipitates_

Chromatography

• TLC Method:
  • Use gloves, apply sample, develop with solvent
  • Determine (Rf) values for identification

Redox Titrations

• Example with manganate and ethanedioic acid
• Autocatalysis: Mn²⁺ acts as catalyst

Spectrophotometry

• Used to measure concentration of colored ions
• Calibration: Use solutions of known concentration

General Procedures

• Uncertainty:
  • Calculate % error, reduce using better equipment
• Filtration: Use of Buchner flask for vacuum filtration
• Reflux and Distillation:
  • Used for organic reactions; never seal the condenser

Conclusion

• The guide covers a wide range of practical skills essential for A-level chemistry experiments, including safety measures, detailed methodologies, and examples of calculations and experimental procedures.