The s-Block Elements

Question 205
CBSEENCH11007183

Describe the importance of the following: (i) limestone (ii) cement (iii) plaster of paris.

Solution

Importance of limestone
It is used in the manufacture of quick lime.
CaCO3 --> CaO +CO2
Quicklime, when added to water in limited amount, is used to produce slaked lime or milk of lime Ca(OH)2
CaO +H2O ---> Ca(OH)2
It is used as a flux to remove acidic impurities
CaO +SiO2 ---> CaSiO3
6CaO + P4O10--> 2Ca3(PO4)2

The importance of cement: Cement is an important building material. it is used in the concrete and reinforced concrete, in plastering and in the construction of dams, bridges and buildings.

The importance of plaster of Paris:On heating gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate plaster of Paris.Plaster of Paris is a white powder and on mixing with water, it changes to gypsum once again giving a hard solid mass. It is used for immobilising the affected part where there is a bone fracture. It is used for making statues, models and other decorative materials.

Question 206
CBSEENCH11008047

The main oxides formed on combustion of Li, Na and K in excess of air are, respectively:

  • LiO2, Na2O2 and K2O

  • LiO2, Na2O2 and KO2

  • LiO, Na2O2 and KO2

  • LiO, Na2O and KO2

Solution

C.

LiO, Na2O2 and KO2

2 Li space plus space 1 half straight O subscript 2 space left parenthesis straight g right parenthesis space rightwards arrow Li subscript 2 straight O
2 Na space plus space straight O subscript 2 left parenthesis straight g right parenthesis space rightwards arrow Na subscript 2 straight O subscript 2
straight K space plus space straight O subscript 2 space left parenthesis straight g right parenthesis space rightwards arrow space KO subscript 2 space left parenthesis straight g right parenthesis
Question 207
CBSEENCH11008057

Which one of the following alkaline earth metal sulphates has its hydration enthalpy greater than its lattice enthalpy?

  • CaSO4

  • BeSO4

  • BaSO4

  • SrSO4

Solution

B.

BeSO4

Hydration energy basically depends on the size of an atom.
(i) Smaller the size atom greater the hydration energy.
(ii) Be is small in size thus have greater hydration energy.

Question 208
CBSEENCH11008064

The correct statement for the molecule, CsI3 is 

  • It is a covalent molecules

  • It contains Cs+ and I3-

  • It contains Cs3+ and I- ions

  • It contains Cs+, I- and lattice I2 molecule

Solution

B.

It contains Cs+ and I3-

I3- is an ion made up of I2 and I- which  has linear shape
While Cs+ is an alkali metal cation.

NCERT Book Store

NCERT Sample Papers

Entrance Exams Preparation