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The Solid State

Short Answer Type

231. With reference to crystal structure, what is meant by coordination number?

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232. What is the coordination number of atoms
(i) in a cubic close packed structure?
(ii) in a body-centred cubic structure?

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233. Calculate the efficiency (percentage of volume occupied and unoccupied) of packing in case of a metal crystal for simple cubic.

Let ‘a’ be the edge length of the unit cell and r be the radius of sphere.

As sphere are touching each other

Therefore a = 2r

No. of spheres per unit cell = 1/8 × 8 = 1

Volume of the sphere = 4/3 πr3

Volume of the cube = a3= (2r)3 = 8r3

∴ Fraction of the space occupied = 1/3πr3 / 8r3 = 0.524

∴ % occupied = 52.4 %

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234.

(a) Determine the type of cubic lattice to which a given crystal belongs if it has edge length of 290 pm and density is 7.80 g cm^–3. (Molecular mass.= 56 g mol^–1)

(b) Why does zinc oxide exhibit enhanced electrical conductivity on heating?

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235. The compound CuCl has the ZnS (Cubic) structure. Its density is 3.04 g cm^–3. What is the length of the edge of unit cell? (At. mass of Cu = 63.5, CI = 35.5)

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236. Potassium crystallises in a simple cubic unit cell. It has an atomic mass of 209 and its density is 91.5 g m^–3. What is the edge length of its unit cell?

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237. An element crystallises in fcc structure with an edge of 200 pm. Calculate density if 200 g of this element contains 24 x 10²³ atoms.

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238. The nearest neighbour Ag atoms in the silver crystal are 2.87 x 10^–10 m apart. What is the density of silver? Silver crystallises in fcc form.

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239. The cell edge of a fcc element of atomic mass 108 is 409 pm. Calculate its density.

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240. An element ‘A’ of atomic mass 100 having bcc structure has unit cell edge of 400 pm. Calculate the density of ‘A’ and the number of unit cells for 10 g of ‘A’.

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