1
MCQ (Single Correct Answer)

AIPMT 2008

Which of the following statements is not correct?
A
The number of carbon atoms in a unit cell of diamond is 4
B
The number of Bravais lattices in which a crystal can be categorized is 14
C
The fraction of the total volume occupied by the atoms in a primitive cell is 0.48.
D
Molecular solids are generally volatile.

Explanation

Packing fraction for a cubic unit cell is given by

f = $${{Z \times {4 \over 3}\pi {r^3}} \over {{a^3}}}$$

where a = edge length, r = radius of cation and anion.

Efficiency of packing in simple cubic or primitive cell = π/6 = 0.52 or 52 %

It means 52 % of unit cell is occupied by atoms and 48 % is empty.
2
MCQ (Single Correct Answer)

AIPMT 2008

With which one of the following elements silicon should be doped so as to give p-type of semiconductor?
A
Selenium
B
Boron
C
Germanium
D
Arsenic

Explanation

The semiconductors formed by the introduction of impurity atoms containing one elecron less than the parent atoms of insulators are termed as p-type semiconductors. Therefore silicon containing 14 electrons has to be doped with boron containing 13 electrons to give a p-type semi-conductor.
3
MCQ (Single Correct Answer)

AIPMT 2007

If NaCl is doped with 10$$-$$4 mol% of SrCl2, the concentration of cation vacancies will be (NA = 6.02 $$ \times $$ 1023 mol$$-$$1)
A
6.02 $$ \times $$ 1016 mol$$-$$1
B
6.02 $$ \times $$ 1017 mol$$-$$1
C
6.02 $$ \times $$ 1014 mol$$-$$1
D
6.02 $$ \times $$ 1015 mol$$-$$1

Explanation

As each Sr2+ ion introduces one cation vacancy, therefore concentration of cation vacancies = mole % of SrCl2 added.

$$ \therefore $$ Concentration of cation vacancies

= 10–4 mole %

= $${{{{10}^{ - 4}}} \over {100}} \times 6.023 \times {10^{23}}$$

= $$6.023 \times {10^{23}}$$ $$ \times $$ 10-6

= $$6.023 \times {10^{17}}$$
4
MCQ (Single Correct Answer)

AIPMT 2007

The fraction of total volume occupied by the atoms present in a simple cube is
A
$${\pi \over {3\sqrt 2 }}$$
B
$${\pi \over {4\sqrt 2 }}$$
C
$${\pi \over 4}$$
D
$${\pi \over 6}$$

Explanation

The maximum properties of the available volume which may be filled by hard sphere in simple cubic arrangement is $${\pi \over 6}$$ or 0.52.

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