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Chemistry Core/AP

Lab #1: Hydrate Composition

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Laboratory Exercise

Goal: to determine the ratio of water to magnesium phosphate in Epsom salts.



  1. Carefully clean and dry the test tubes and reject any with scratches or cracks.
  2. Weight the test tubes as accurately as possible.
  3. Place approximately 3 grams of Epsom salts in the tube. Weigh the tube and salt together and record the amount of Epsom salts as accurately as possible.
  4. Using proper techniques for handling the tube with tongs, heat the salt gradually to drive off water for 1-2 minutes.
  5. Allow the tube to cool, and weigh it and the remaining salt together (do not remove the salt!). Record the new weight.
  6. Reheat the tube for 1-2 minutes, cool and weigh.
  7. Repeat the heat-cool-and-weigh cycle until the weight of the tube and remaining salt stops changing.
  8. Record the final weight.
  9. Determine the mass due to the remaining Epsom salt.
  10. Determine the mass of water driven off by heating the salt.
  11. Determine the percentage of mass in the original sample due to water.
  12. Given that anhydrous Epsom salt has the formula MgSO4, determine the molecular formula for Epsom salts based on your data.


Describe your procedure, especially the methods you used to weigh the tubes and salts. Present all of your data and calculations in an orderly manner, and show how you determined the moleuclar formula for Epsom salt.