The density of KBr is 2.75 g cm^-3 length of the unit cell is 654 pm. K = 39, Br = 80, then what is true about the predicted nature of the solid:
1) Solid has face centred cubic system with co-ordination number = 6
2)Solid has simple cubic system with co-ordination number = 4
3) Solid has face centred cubic system with co-ordination number = 1
4) None of these

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Explanation:

The Solid State

Packing Efficiency and Calculations Involving Unit Cell Dimensions

The density of KBr is 2.75 ...

The density of KBr is 2.75 g cm

−3

. The length of the unit cell is 654 pm. Atomic mass in amu of K=39,Br=80. Then, the solid is:

MEDIUM

ANSWER

The expression for the density of the unit cell is as given below:

d=

V×N

o

z×m

=

a

3

×N

o

z×m

Hence, z=

m

d×a

3

×N

o

Substituting values in the above expression, we get

z=

119

2.75×(654×10

−10

)

3

×6.023×10

23

⟹n=4

The unit cell contains 4 molecules of KBr.

Hence, it is a face-centered cubic unit cell.

Hence, the correct option is A

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