Question:
3.786g of hydrated zinc sulphate, ZnSO4•xH2O, is heated to remove all water of crystallisation. The mass of anhydrous ZnSO4 formed is 2.122g. What is the formula of the hydrated zinc sulphate?
Answer:
Water of crystallisation describes water molecules that are bonded into a crystalline structure of a compound. By knowing the mass of the hydrated salt (3.786g) and of the anhydrous salt (2.122g) it is possible to work out the formula of the hydrated salt. First, write out the chemical equation for the reaction: ZnSO4•xH2O(s) → ZnSO4 (s) + xH2O(l)Next, work out the mass of the water (mass of the hydrated salt-mass of the anhydrous salt): 3.786g-2.122g= 1.664gThen, work out the number of moles of water and of the anhydrous salt using n=m/M: Number of moles of water: 1.664g/18gmol-1= 0.092444molFormula mass of the anhydrous salt: 65.4+32.1+(16×4)= 161.5gmol-1Number of moles of the anhydrous salt: 2.122g/161.5gmol-1 =0.013139molFinally, work out the ratio of the number of moles of water to the number of moles of the anhydrous salt and so X: Ratio: 0.092444mol/0.013139mol= 7.04 Therefore X= 7 and the formula of the hydrated salt is ZnSO4•7H2O
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