1

### AIPMT 2006

CsBr crystallises in a body centered cubic lattice. The unit cell length is 436.6 pm. Given that the atomic mass of Cs = 133 and that of Br = 80 amu and Avogadro number being 6.02 $\times$ 1023 mol$-$1, the density of CsBr is
A
4.25 g/cm3
B
42.5 g/cm3
C
0.425 g/cm3
D
8.25 g/cm3

## Explanation

Density of CsBr = ${{Z \times M} \over {V \times {N_A}}}$

= ${{1 \times 213} \over {{{\left( {436.6 \times {{10}^{ - 10}}} \right)}^3} \times 6.023 \times {{10}^{23}}}}$

= 4.25 g/cm3
2

### AIPMT 2006

The appearance of colour in solid alkali metal halides is generally due to
A
interstitial positions
B
F.-centres
C
Schottky defect
D
Frenkel defect

## Explanation

F-centres are the sites where anions are missing and instead electrons are present. They are responsible for colours.
3

### AIPMT 2005

In a face-centered cubic lattice, a unit cell is shared equally by how many unit cells?
A
2
B
4
C
6
D
8

## Explanation

In given unit cell it is shared equally by six faces of different unit cells.
4

### AIPMT 2004

A compound formed by elements X and Y crystallises in a cubic structure in which the X atoms are at the face-centres. The formula of the compound is
A
XY3
B
X3Y
C
XY
D
XY2

## Explanation

X atoms at the corners = ${1 \over 8} \times 8$ = 1

Y atoms at the face centres = ${1 \over 2} \times 6$ = 3

Ratio of atoms, X : Y = 1 : 3

Hence, formula is XY3.