Which one of the following is the number of water molecules that share with two formula unit CaSO₄ in plaster of Paris ?
- A.One
- B.Two
- C.Five
- D.Ten
Correct answer
Explanation
The correct answer is A, One. Plaster of Paris is calcium sulphate hemihydrate, CaSO₄·½H₂O, so one molecule of water is shared by two formula units of calcium sulphate, written (CaSO₄)₂·H₂O.
It is made by heating gypsum, CaSO₄·2H₂O, to about 373 K (100 °C), when gypsum loses one and a half molecules of water per formula unit. When plaster of Paris is mixed with water it takes the water back, turns into gypsum again and sets into a hard mass. This is why it is used for setting fractured bones, making toys, statues and false ceilings. It must be stored in moisture-proof containers.
B is wrong because two water molecules per formula unit belong to gypsum, not plaster of Paris.
C is wrong because five water molecules are found in blue vitriol, CuSO₄·5H₂O.
D is wrong because ten water molecules are found in washing soda, Na₂CO₃·10H₂O.
Exam tip: gypsum CaSO₄·2H₂O → heat → plaster of Paris CaSO₄·½H₂O → add water → gypsum again.
Practice Questions
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- B.fission and thermonuclear fusion
- C.geothermal fission and fusion
- D.geothermal fusion and fission
Show answer
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- B.Synergid
- C.Egg
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- A.all the three regions (the emitter, the base and the collector) have equal concentrations of impurity
- B.the emitter has the least concentration of impurity
- C.the collector has the least concentration of impurity
- D.the base has the least concentration of impurity
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- A.Pyrex glass
- B.Soft glass
- C.Hard glass
- D.Flint glass
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- A.Nucleus
- B.Endoplasmic reticulum
- C.Golgi bodies
- D.Ribosomes
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