The density of potassium bromide crystal...

The density of potassium bromide crystal is `2.75 g cm^(-3)` and the length of an edge of a unit cell is 654 pm. The unit cell of KBr is one of three types of cubic unit cells. How many formula units of KBr are there in a unit cell ? Does the unit cell have a NaCl or CsCl structure ?

Text Solution

Verified by Experts

We know that,
`rho=(N)/(a^(3))[(M)/(N_(A))]`
`N=(rhoxxa^(3)xxN_(A))/(M)`
`=(2.75xx(654xx10^(-10))^(3)xx6.023xx10^(23))/(119)=3.89 ~~4`
Number of mass points per unit cell =4
It is NaCl type crystal, i.e., fcc structure.

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The formula for determination of density of cubic unit cell is :

`(a^(3)N_(0))/(ZxxM)g cm^(-3)`

`(ZxxN_(0))/(Mxx a^(3))g cm^(-3)`

`(a^(3)xxM)/(ZxxN_(0))g cm^(-3)`

`(Z xx M)/(N_(0)xx a^(3))g cm^(-3)`

The density of KBr is 2.75g cm^(-3) . The edge length of unit cell is 654 pm, then structure of KBr, is: ( M_(w) of KBr=119gmol^(-1) )

BCC type

SCC type

FCC type

HCP type

The density of KBr is 2.75 g cm^(-3) . The length of the unit cell is 654 pm. Atomic mass of K = 39, Br = 80 . Then what is true about the predicted nature of the solid?

The unit cell is fcc.

`Z = 4`.

There are four `"constituents"//"unit"` cells.

There are `8` ions at corners and `6` at the centres of the faces.