1

### 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.
2

### AIPMT 2003

The pyknometric density of sodium chloride crystal is 2.165 $\times$ 103 kg m$-$3 while its X-ray density is 2.178 $\times$ 103 kg m$-$3. The fraction of unoccupied sites in sodium chloride crystal is
A
5.96
B
5.96 $\times$ 10$-$2
C
5.96 $\times$ 10$-$1
D
5.96 $\times$ 10$-$3

## Explanation

Molar volume from pyknometric density

= ${M \over {2.165 \times {{10}^3}}}$ m3

Molar volume from X-ray density

= ${M \over {2.178 \times {{10}^3}}}$ m3

Volume unoccupied

= ${M \over {{{10}^3}}}\left( {{1 \over {2.165}} - {1 \over {2.178}}} \right)$ m3

Fraction unoccupied

= ${{\left( {{{0.013M \times {{10}^{ - 3}}} \over {2.165 \times 2.178}}} \right)} \over {\left( {{{M \times {{10}^{ - 3}}} \over {2.165}}} \right)}}$

= 5.96 $\times$ 10-3
3

### AIPMT 2001

When Zn converts from melted state to its solid state, it has hcp structure, then find the number of nearest atoms.
A
6
B
8
C
12
D
4

## Explanation

hcp is a closed packed arrangement in which the unit cell is hexagonal and coordination number is 12.
4

### AIPMT 2000

Cation and anion combines in a crystal to form following type of compound
A
ionic
B
metallic
C
covalent
D
dipole-dipole.

## Explanation

We know that electrostatic force is that binds the oppositely charged ions which are formed by transfer of electron from one atom to another is called ionic bond. We also know that cation and anion are oppositely charged particles therefore they form ionic bond in crystal.