Question

`A_(2)B` molecules`(" molar mass " = 259.8 g//"ml")` crystallises in a hexagonal lattice as shown in figure .The lattic constants were `a=5 Å` and `b=8 Å` . If denstiy of crystal is `5 g//cm^(3)` then how many molecules are contained in given unit cell ? `(" Use" N_(A) = 6xx10^(23))`

A. 6
B. 4
C. 3
D. 2

Answer 1

Correct Answer - D
Volt. Of unit cell `=a^(2) sin 60^(@)xxb=173.2xx10^(-24)cm^(3)`
`a=5overset(@)A`
Mass of unti cell=
`173.2xx10^(-24)xx5xx6xx10^(23)=519.6g`
No. of molecules present in given unit cell `=(519.6)/(259.8)=2`