Which one of the following statements is NOT correct?
- (a)Water of crystallization is the fixed number of water molecules present in one formula unit of a salt
- (b)The molecular formula of Plaster of Paris is CaSO4·2H2O
- (c)Washing soda is used for removing the permanent hardness of water
- (d)The atmosphere of Venus is made up of thick white and yellow clouds of sulphuric acid
Correct — B, this is the statement that is NOT correct. Plaster of Paris is calcium sulphate hemihydrate, CaSO4·½H2O (equivalently 2CaSO4·H2O). The formula CaSO4·2H2O, with two water molecules, is gypsum — the mineral from which Plaster of Paris is made by heating. So calling CaSO4·2H2O the molecular formula of Plaster of Paris misassigns gypsum's formula to it. The other three statements are correct.
- (a)Water of crystallization is the fixed number of water molecules present in one formula unit of a salt — This is correct as stated — water of crystallization is exactly the fixed number of water molecules chemically combined in one formula unit of a crystalline salt (for instance, five in CuSO4·5H2O).
- (c)Washing soda is used for removing the permanent hardness of water — This is correct — washing soda (sodium carbonate, Na2CO3·10H2O) removes the permanent hardness of water by precipitating the dissolved calcium and magnesium salts as insoluble carbonates. So it is a true statement, not the answer.
- (d)The atmosphere of Venus is made up of thick white and yellow clouds of sulphuric acid — This is correct — Venus is wrapped in thick clouds of sulfuric acid that appear white to yellow, a standard fact about its atmosphere.
Plaster of Paris (POP) is calcium sulphate hemihydrate, CaSO4·½H2O. It is made by heating gypsum (CaSO4·2H2O) to about 373 K (100°C), which drives off three-quarters of the water. On adding water, POP sets back into a hard mass of gypsum, which is why it is used for casts and moulds. Confusing the two hydrates is the classic error.
This is a 'find the false statement' item where three statements are textbook-true and one swaps a formula. Knowing that gypsum has two water molecules while POP has half a water molecule immediately isolates (b) as the wrong one.
- Plaster of Paris is calcium sulphate hemihydrate, CaSO4·½H2O.
- Gypsum is CaSO4·2H2O; heating gypsum to about 373 K (100°C) gives Plaster of Paris.
- Water of crystallization is the fixed number of water molecules in one formula unit of a hydrated crystal (for example CuSO4·5H2O).
- Venus's atmosphere carries thick clouds of sulfuric acid, giving the planet a pale yellowish-white appearance.
- Swapping the formulae of gypsum (CaSO4·2H2O) and Plaster of Paris (CaSO4·½H2O).
- Forgetting that washing soda removes permanent hardness (temporary hardness is removed by boiling or by adding slaked lime).
Asked as a 'which statement is NOT correct' set that mixes chemistry formulae with a general-science fact.
Consider the following statements: 1. Baking soda is used in fire extinguishers. 2. Quicklime is used in the manufacture of glass. 3. Gypsum is used in the manufacture of Plaster of Paris. Which of the statements given above is/are correct?
- (a) 1 and 2
- (b) 2 and 3
- (c) 1 only
- (d) 1, 2 and 3
Answer(d) 1, 2 and 3
Same gypsum–Plaster of Paris relationship — statement 3 there ('gypsum is used to make Plaster of Paris') is exactly the fact this NDA item hides, since POP is CaSO4·½H2O made by heating gypsum (CaSO4·2H2O).
- practice — not a real PYQ
The chemical formula of Plaster of Paris is
- (a)CaSO4·2H2O
- (b)CaSO4·½H2O
- (c)CaSO4
- (d)Ca(OH)2
Answer(b) CaSO4·½H2O — calcium sulphate hemihydrate.
- practice — not a real PYQ
Plaster of Paris is obtained by heating gypsum to about
- (a)100°C (373 K)
- (b)500°C
- (c)0°C
- (d)1000°C
Answer(a) about 100°C (373 K), which drives off most of the water of crystallization.