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Density

The density o...

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 $K B r$ is one of the three types of cubic unit cell. How many of formula units of $K B r$ are there in a unit cell? $[K B r = 119]$

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Solution

The correct option is B $4$
Given that,
Density, $ρ = 2.75$ $g / c m^{3}$
Edge length, $a = 6.54 ˚ A$
Molar mass, $M_{A} = 119$
$Z = ?$

Using density formula,
Density, $ρ = \frac{Z \times M_{A}}{N_{A} \times a^{3}}$
$⟹ 2.75 = \frac{Z \times 119}{6.023 \times 10^{23} \times 279 \times 10^{- 24}}$
$⟹ Z \approx 4$

There are $4$ formula units of $K B r .$

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