This set of Chemistry Multiple Choice Questions & Answers (MCQs) focuses on “s-Block Elements – Important Compounds of Calcium”.
1. Which of the following acid chemical formula first slaked lime?
a) Calcium chloride
b) Calcium oxide
c) Calcium hydroxide
d) Calcium carbonate
Explanation: When 1 mole of burnt lime, that is calcium oxide is combined with 1 mole of the hydrogen molecule, a hissing sound appears and slaked lime is produced along with the heat. The chemical formula for slaked lime Ca(OH)2.
2. Quicklime as same as calcium oxide.
Explanation: Yes, calcium oxide chemical formula is CaO and it is also called as quicklime or simply lime. It is prepared by the thermal decomposition of calcium carbonate at 1072-1270 Kelvin, carbon dioxide comes as a byproduct and it is a basic oxide.
3. Which of the following is not a compound of mortar?
Explanation: Quicklime or calcium oxide (CaO) is used as a basic flux, for removing the hardness of water, also used in mortar. A mixture of quick lime and sand in the ratio 1:3 with enough water to make a thick place is called motor.
4. What is the enthalpy of heat for dissolving quicklime and water?
a) 55 kJ/mol
b) 63 kJ/mol
c) 75 kJ/mol
d) 78 kJ/mol
Explanation: Calcium hydroxide is prepared by dissolving quicklime and water and the enthalpy change for this reaction is 63 kJ/mole (the sign is negative). Here the chemical formulae for quicklime and Calcium hydroxide are CaO and Ca(OH)2 has twice respectively.
5. Which of the following compound is formed when slaked lime is treated with excess dioxide in the presence of water?
a) Barium sulphate
b) Calcium carbonate
c) Calcium hydroxide
d) Calcium bicarbonate
Explanation: When 1 mole of Calcium Hydroxide is treated with an excess of carbon dioxide in the presence of water, it results in the formation of calcium bicarbonate which is soluble. It is one of the most important properties of slaked lime.
6. Limestone is insoluble in water.
Explanation: Calcium carbonate is also known as limestone or marble are chalk, its chemical formula is given by CaCO3. It is insoluble in water but dissolves in the presence of carbon dioxide due to the formation of calcium bicarbonate.
7. Can limestone be prepared through slaked lime?
d) Cannot say
Explanation: Yes, limestone can be prepared through slaked lime. Limestone is calcium carbonate while slaked lime is calcium hydroxide. When limestone is passed through carbon dioxide, it results in the formation of calcium carbonate which is a precipitate and water.
8. Which of the following do you think is the correct formula for gypsum?
Explanation: Gypsum is a compound of calcium. It is chemically calcium sulphate dehydrate and it is also known as alabaster. It is added to cement to slow down at speed of setting. It’s really very essential in our day to day life.
9. Gypsum is the same as plaster of Paris.
Explanation: Plaster of Paris is calcium sulphate hemihydrate and gypsum is calcium sulphate dihydrate. On heating gypsum at 390 Kelvin, it gives plaster of Paris. The chemical formula of gypsum and plaster of Paris is CaSO4.2H2O and CaSO4.1/2H2O respectively.
10. What is dead burnt plaster?
a) Hexahydrate calcium sulphate
b) Hemihydrate calcium sulphate
c) Anhydrous calcium sulphate
d) Decahydrate calcium sulphate
Explanation: When plaster of Paris or calcium sulphate hemihydrate, which is given by the chemical formula CaSO4.1/2H2O is heated about 393 Kelvin, no water of crystallization is left and anhydrous calcium sulphate is formed, it is known as dead burnt plaster.
11. Is the plaster of Paris useful in setting a solid mass?
d) Cannot say
Explanation: When plaster of Paris or calcium sulphate hemihydrate which is given by the chemical formula CaSO4.1/2H2O is mixed with water, it forms of plastic mass which sets into a solid mass with slight expansion due to dehydration and its reverse conversion into gypsum.
12. What is chloride of lime?
a) Calcium sulphate
b) Calcium chloro hypochlorite
c) Calcium chloride
d) Calcium hydroxide
Explanation: Chloride of lime is chemically known as calcium chloro hypochlorite and it is given by the chemical formula CaOCl2, it is also known as bleaching powder. Bleaching powder is prepared by the fusion of calcium hydroxide and chlorine.
13. What is the average percentage of available chlorine theoretically?
Explanation: With an excess of dilute sulfuric acid and carbon dioxide, calcium chloro hypochlorite forms chlorine, which is known as available chlorine. The average percentage of available chlorine is 35 to 40% but theoretically, it should be 49%, which diminishes on keeping the Powder due to a chemical change.
14. Which of the following is not a use of calcium chloro hypochlorite?
a) Sterilization of water
b) Manufacture of chloroform
Explanation: Calcium chloro hypochlorite or chloride of lime or bleaching powder which is chemically CaOCl2 is used for bleaching, as a disinfectant and germicide in the sterilization of water, for making wool which is unshrinkable and in the manufacture of chloroform.
15. In the average composition of portland cement, the percentage of magnesium oxide is a ___________
a) 3 to 4%
b) 2 to 3%
c) 4 to 5%
d) 1 to 2%
Explanation: Cement is an important building material. The average composition of portland cement is calcium oxide of 50 to 60%, Silicon dioxide of 20 to 25%, aluminium oxide of 5 to 10%, magnesium oxide of 2 to 3%, ferric oxide of 1 to 2% and sulphur trioxide of 1 to 2%.
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