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Anhydrous and hydrated salts

Water of crystallisation Lesson objectives At the of the lesson you should be able…. a. Explain the terms anhydrous, hydrated and water of crystallisation. b. calculate the formula of a hydrated salt using percentage composition, mass composition or experimental data.

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Anhydrous and hydrated salts

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  1. Water of crystallisationLesson objectivesAt the of the lesson you should be able….a. Explain the terms anhydrous, hydrated and water of crystallisation.b. calculate the formula of a hydrated salt using percentage composition, mass composition or experimental data

  2. Most salts exist in the solid form either as pure substance or in the form of crystals that contain water molecules as part of their structure.An anhydrous salt is refers to a salt that contains no water molecules. Example: Sodium sulphate Na2SO4A hydrated salt refers to a salt(crystalline compound) containing water molecules.Examples CuSO4.5H2O Copper sulphate pentahydrate CoCl2.6H2O Cobalt chloride hexahydrate FeSO4.7H2O Iron sulphate heptahydrate Na2SO4.10H2O Sodium sulphate decahydrate Anhydrous and hydrated salts

  3. EMPIRICAL VS DOT FORMULAE Note: The relative number of water molecule in a hydrated salt is indicated by a number after the dot. Thus, CuSO4.5H2O has 5 molecules of water of crystallisation.

  4. Determination of the formula of hydrated saltThe formula of a hydrated salt can be determined by heating so that the water of crystallisation evaporates completely.The following are determined from the results:a. the mass of the hydrated salt, containing water of crystallisationb. the mass of the anhydrous salt, without the waterc. the mass of water that was in the hydrated salt From the above you then calculate:1. The amount, in mol, of water2. The amount, in mol, of the anhydrous salt3. The mole ratio of water and the anhydrous salt, followed by the simplest ratio.Finally, Write the formula of the hydrated salt by using the ratio.

  5. Example1. A sample of copper crystals has a mass of 6.80 g. The sample is heated to drive off all the water of crystallisation. The mass is reduced to 4.35 g. Calculate the formula of the copper sulphate crystals.Answermass of hydrated salt = 6.80 gmass of anhydrous salt = 4.35 gmass of water driven off by heating = 6.80 – 4.35 = 2.45 gamount , in mol, of water of crystallisation, n =m = 2.45 = 0.136 molM 18amount, in mol, anhydrous copper sulphate(CuSO4), n = mM = 4.35 = 4.35 = 0.02726 mol 63.5+32.1+(16x4) 159.6mole ratioCuSO4 : H2O = 0.02726 : 0.136simplest ratio 0.02726 = 1 0.136 = 5 i.e. 1:5 0.02726 0.02726Therefore, the formula of the hydrated copper sulphate is CuSO4.5H2O

  6. A sample of magnesium crystals contains 9.78% Mg, 38.69% SO42-, and 51.53% H2O by mass. Determine the formula of the crystals.( S = 32.1, H = 1.0, O = 16.0, Mg = 24.3 )ANSWERMole ratioamount, in mol, of magnesium, n = m = 9.78 = 0.4025 molM 24.3amount, in mol, of sulphate, n = m = 38.69 = 0.4026 molM 96.1 amount, in mol, of water, n = m = 51.53 = 2.863 molM 18 simplest ratio 0.4025 = 1 0.4026 = 1 2.863 = 7 0.4025 0.4026 0.4025Therefore, the formula of the magnesium sulphate crystals is MgSO4.7H2O

  7. Now try question 1 – 2 Page 27

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