Water of Crystallisation - UNSOLVED PRACTICE SET

Q7. Define water of crystallisation. Give two examples of salts that contain water of crystallisation, mentioning the number of water molecules in each.

Q8. What happens when hydrated copper sulphate (CuSO₄·5H₂O) is heated? Write the chemical equation. What happens when water is added to the anhydrous salt obtained?

Q9. Your mother notices that the washing soda (Na₂CO₃·10H₂O) she bought last month has become powdery and lost weight. Explain this phenomenon and write the chemical equation involved.

Q10. Differentiate between efflorescence and deliquescence with one example of each.

Q11. Why do crystals of copper sulphate turn white on heating but regain their blue colour when water is sprinkled on them? Explain the role of water of crystallisation.

Q12. Write the chemical formulae of: (i) Hydrated copper sulphate, (ii) Hydrated calcium sulphate (gypsum), (iii) Hydrated sodium carbonate (washing soda), (iv) Hydrated magnesium sulphate (Epsom salt).

Q13. Define water of crystallisation and explain its importance in the structure and properties of crystalline salts. Describe what happens when hydrated copper sulphate crystals are heated, including the colour change and the chemical equation. What happens when water is added to the anhydrous salt? Explain the terms efflorescence and deliquescence with examples.

Q14. A student is given four salts: CuSO₄·5H₂O, Na₂CO₃·10H₂O, CaSO₄·2H₂O, and NaCl. She heats each salt in a test tube and records her observations:

(a) Predict the observations for each salt upon heating.

(b) Write chemical equations for the reactions that occur upon heating.

(c) Which salt will show the greatest percentage loss in mass upon heating? Calculate this percentage for each salt that loses water.

(d) What happens when the heated salts are allowed to cool in open air?

(e) Why is NaCl unaffected by heating in this experiment?

Q15. "Water of crystallisation is not just trapped water — it is an integral part of the crystal structure that determines the colour, shape, and stability of many salts." Discuss this statement by explaining: (i) how water molecules are arranged in copper sulphate crystals, (ii) why the colour of hydrated copper sulphate is blue while anhydrous copper sulphate is white, (iii) the economic importance of understanding water of crystallisation in industries like pharmaceuticals and construction, and (iv) how efflorescence and deliquescence affect the storage and use of salts.

Q16. A chemistry teacher asks students to calculate the percentage of water of crystallisation in various salts:

Table

Salt Formula Molar Mass (g/mol)

Blue vitriol CuSO₄·5H₂O 249.5

Gypsum CaSO₄·2H₂O 172

Washing soda Na₂CO₃·10H₂O 286

Epsom salt MgSO₄·7H₂O 246

(a) Calculate the percentage of water of crystallisation in each salt.

(b) Arrange the salts in order of increasing percentage of water of crystallisation.

(c) If 50 g of blue vitriol is heated to remove all water of crystallisation, calculate the mass of anhydrous CuSO₄ obtained.

(d) If the anhydrous CuSO₄ is then used to test for water in ethanol, and it regains its blue colour, what mass of water has it absorbed?

(e) Why is the knowledge of water of crystallisation important for chemists when weighing salts for reactions?

Q17. A pharmaceutical company uses hydrated salts in the preparation of medicines.

(a) If the company requires 100 g of anhydrous copper sulphate for a formulation, calculate the mass of CuSO₄·5H₂O it needs to weigh out. (Molar masses: CuSO₄ = 159.5 g/mol, CuSO₄·5H₂O = 249.5 g/mol)

(b) If a batch of washing soda (Na₂CO₃·10H₂O) effloresces and loses 60% of its water of crystallisation, calculate the mass of Na₂CO₃·10H₂O needed to obtain 106 g of Na₂CO₃.

(c) The company stores magnesium sulphate (Epsom salt) for medicinal use. If 24.6 g of MgSO₄·7H₂O is heated, calculate the mass of water lost and the mass of anhydrous MgSO₄ remaining. (Molar masses: MgSO₄·7H₂O = 246 g/mol, MgSO₄ = 120 g/mol, H₂O = 18 g/mol)

(d) Why is it important for pharmaceutical companies to know the exact water content in hydrated salts used in medicines?

Q18. In India, many households use alum (KAl(SO₄)₂·12H₂O) for purifying water.

(a) Calculate the percentage of water of crystallisation in alum. (Molar mass of alum = 474 g/mol)

(b) If a family uses 10 g of alum daily to purify 20 litres of water, calculate the mass of pure alum (without water) they consume in a year.

(c) When alum is added to muddy water, it forms a gelatinous precipitate of aluminium hydroxide that entraps suspended particles. Write the chemical equation for this reaction.

(d) Why is alum preferred over chemical coagulants in rural India for water purification?

(e) If 100 kg of alum is transported from a factory, and during transport it loses 5% of its water of crystallisation due to heat, calculate the mass of the remaining salt.