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Calculationsofwaterofcrystallisation

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Calculationsofwaterofcrystallisation

  1. 1. CHEMISTRY CALCULATIONS OF WATER OF CRYSTALLISATION Short revision series
  2. 2. AIM  We are going to learn: 1. How to calculate percentage of water in hydrated salts 2. Percentage of water of crystallisation 1. How to determine the formula of a salt from the water of crystallisation
  3. 3. How to calculate percentage of water in hydrated salts  Question: Calculate the % of water in hydrated MgSO4.5H2O salt crystals. (take relative atomic masses of Mg = 24, S=32, O=16, H = 1)  SOLUTION:  - First calculate the relative molecular mass of MgSO4.5H2O = 24 + 32 + (4 x 16) + (5 x (1 + 1 +16) = 210
  4. 4. Solution cont. Next, calculate the mass of 5H2O = 5 x 18 = 90 Therefore, the % of water is = mass of water x 100 mass of MgSO4.5H2O = 90/210 x 100 = 42.86%
  5. 5. How to calculate % of water of crystallisation  Question: Calculate the percentage of water of crystallisation if 6.25g of CuSO4.xH2O (x unknown) was heated gently until a mass of 4.00g is remaining. (take relative atomic masses of Cu = 64, S=32, O=16, H = 1)  SOLUTION:  First calculate the mass difference = 6.25 – 4 = 2.25g (this is the mass of the water of crystallisation)  % of water of crystallisation = 2.25/6.25 x 100 = 36%
  6. 6. How to determine the formula of a salt from the water of crystallisation  Question: 6.25g of CuSO4.xH2O (x unknown) was heated gently until a mass of 4.00g is remaining. What is the formula of the salt? (take relative atomic masses of Cu = 64, S=32, O=16, H = 1)  SOLUTION After heating the mass of salt left is 4.00g, while mass of water of crystallisation is mass that was given off or lost, i.e 6.25 – 4 = 2.25g
  7. 7. SOLUTION CONT The mass ratio CuSO4 and H2O = 4 : 2.25 Convert from mass ratio to mole ratio by dividing the mass by their molecular mass of each Molecular mass of CuSO4 =160; while H2O=18 Mole ratio CuSO4 : H2O =4/160 : 2.25/18 = 0.025 : 0.125. *(divide by the smallest i.e 0.025) = 1 : 5 So the formula of the salt is CuSO4.5H2O

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