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 Multiple Choice QuestionsShort Answer Type

241. Aluminium forms fcc cubic crystals. The density of aluminium is 2.7 g cm–3. Calculate the length of the edge of the unit cell of Al.
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242. The density of KBr is 2.75 g cm–3. The length of the edge of the unit cell is 654 pm. Show that KBr has a fcc structure.
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243. Tungsten has a density of 19.35 g cm–3 and the length of the side of the unit cell is 316 pm. The unit cell is a body centred unit cell. How many atoms does 50 grams of the element contain ?
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244. In the cubic crystal of CsCl (d = 3.97 g cm–3), the eight corners are occupied by CIwith a Cs+ at the centre and vice versa. Calculate the distance between the neighbouring Cs+ and CI ions. What is the radius ratio of the two ions ? (At. mass of Cs = 132.91 and CI = 35.45).
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 Multiple Choice QuestionsLong Answer Type

245. How the crystalline solids are classified on the basis of the nature of bonding? Give suitable examples and nature of the forces present in different types of solids.
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246. Write two important features, coordination numbr of the ions and number of formula units per unit cell for the following crystals:
(i) Cesium chloride, (ii) Zinc sulphide, (iii) Calcium fluorite, (iv) Sodium oxide.


i) CsCl is simple cubic cell. Cesium ion is surrounded by eight chloride ion which are also disposed towards the corner of a cube therefore both type ions are in equivalent positions and the stoichiometry is 1:1 . the coordination of CsCl is 8:8.

ii) Zinc sulfide is a FCC unit cell. The net number of zinc cation per unit cell is four, and the net number of sulfide anions per unit cell is four therefore, the ratio of ZnS ion in the cell is 1:1.

iii)Calcium fluoride is a FCC unit cell. The net number of Calcium cation per unit cell is four and the net number of fluoride anion is eight. Therefore the ratio of CaF2 ion in cell is 1:2 

iv) Na2O has the structure opposite to CaF2. In
this case coordination number of Na+ ions is 4 and that of O2- ion is 8. Thus Na2O has 4:8 coordination.



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247. Explain the reasons why:
(a) Frenkel defect is not found in pure alkali halides?
(b) Zinc oxide appears yellow on heating?
(c) Solid containing F-centres are para magnetic?
(d) Uncharged atoms or molecules never crystallize in a simple cubic structure?
(e) A given element will have the same density in both a hexagonal close-packed structure and a cubic close-packed structure?
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 Multiple Choice QuestionsShort Answer Type

248.

What is the formula of a compound in which the element Y forms ccp lattice and atoms of X occupy 1/3rd of tetrahedral voids?

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249.

An element with molar mass 27 g mol-1  forms a cubic unit cell with edge length 4.05 x 10-8 cm . If its density is 2.7 g cm-3 , what is the nature of the cubic unit cell?

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250.

An element with density 11.2 g cm-3  forms a f.c.c. lattice with edge length of 4 x10-8

Calculate the atomic mass of the element. (Given:  NA = 6.022x 10-23 (mol-1

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