Practical Chemistry

Experiment 13: Hess' law


To use Hess' law to find the hydration enthalpy of MgSO4


Hydration enthalpy can be defined as the energy change when 1 mole of an anyhydrous salt forms 1 mole of a hydrated salt:

MgSO4 + 7H2O MgSO4.7H2O


  1. Measure approximately 50ml of water into a weighed polystyrene cup. Re-weigh.
  2. Record the temperature over 1 minute to ensure stability.
  3. Add approximately 5 g of anhydrous magnesium sulphate (you must know the mass exactly)
  4. Stir to dissolve noting the highest temperature reached.
  5. Repeat the procedure using hydrated magnesium sulphate in place of anhydrous magnesium sulphate.

Experimental report

The following factors must be considered:

  • Accurate recording of raw data
  • Choice of suitable apparatus and method
  • Presentation of raw data
  • Data analysis and presentation
  • Evaluation of errors both qualitative and numerical

Assessment suitable for:

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