Solid State

NEET Chemistry · 84 questions · Page 9 of 9 · Click an option or "Show Solution" to reveal answer

Q81

Na and Mg crystallize in BCC and FCC type crystals respectively, then the number of atoms of Na and Mg present in the unit cell of their respective crystal is :

A 4 and 2

B 9 and 14

C 14 and 9

D 2 and 4

Correct Answer

Option D

Solution

In bcc - points are at corners and one in the center of the unit cell. Number of atoms per unit cell

= 8 \times {1 \over 8} + 1 = 2.

In fcc - points are at the corners and also centre of the six faces of each cell. Number of atoms per unit cell

= 8 \times {1 \over 8} + 6 \times {1 \over 2} = 4.

Q82

Which of the following exists as covalent crystals in the solid state?

A Silicon

B Sulphur

C Phosphorous

D Iodine

Correct Answer

Option A

Solution

Among the given crystals only silicon is as a covalent solid.

Q83

0.27 g of a long chain fatty acid was dissolved in 100 cm3 of hexane. 10 mL of this solution was added dropwise to the surface of water in a round watch glass. Hexane evaporates and a monolayer is formed. The distance from edge to centre of the watch glass is 10 cm. What is the height of the monolayer? [Density of fatty acid = 0.9 g cm–3,

\pi

= 3]

A 10–8 m

B 10–2 m

C 10–4 m

D 10–6 m

Correct Answer

Option D

Solution

In 100 ml of hexane solution contains 0.27 g of fatty acid.

$\therefore$ In 10 ml of hexane solution contains 0.027 g of fatty acid.

Volume of fatty acid present on the round glass =

{{0.027} \over {0.9}}

As here Area of fatty acid layer = Area of round plate =

\pi {r^2}

$\therefore$ Volume of fatty acid layer =

\pi {r^2}

$\times$ h $\therefore$

\pi {r^2}

$\times$ h =

{{0.027} \over {0.9}}

$\Rightarrow$ 3 $\times$

{\left( {10} \right)^2}

$\times$ h =

{{0.027} \over {0.9}}

$\Rightarrow$ h = 10-4 cm = 10-6 m

Q84

The incorrect statement about the imperfections in solids is :

A Schottky defect decreases the density of the substance.

B Interstitial defect increases the density of the substance.

C Frenkel defect does not alter the density of the substance.

D Vacancy defect increases the density of the substance.

Correct Answer

Option D

Solution

The vacancy defect increases the density of substance.

It does not change the density of the crystal.

It only creates cationic vacancies.

Frenkel Defect causes vacancy defect at its original site and an interstitial defect at its new location.

Therefore, it does not change the density of the solid.

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