Question

The density of potassium bromide crystal is $2.75$ gm$/c{m}^3$ and the length of an edge of the unit cell is $654$ pm. The unit cell of $KBr$ is one of the three types of cubic unit cell. How many of formula units of $KBr$ are there in a unit cell? $[KBr=119]$

• A. $2$
• B. $4$
• C. $6$
• D. $8$

Answer 1

The correct option is B $4$

Given that,

Density, $\rho =2.75$ $g/c{m}^3$

Edge length, $a=6.54\mathring{A}$

Molar mass, $M_A=119$

$Z=?$

Using density formula,

Density, $\rho =\frac{Z\times M_A}{N_A\times a^3}$

$⟹2.75=\frac{Z\times 119}{6.023\times {10}^{23}\times 279\times {10}^{-24}}$

$⟹Z\approx 4$

There are $4$ formula units of $KBr.$