`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))`

Answer

2

Explanation

Volume of unit cell `=a^2 sin 60xxb`
`=173.2xx10^(-24) cm^3`
Mass of unit cell `=173.2xx10^(-24)xx5xx6xx10^(23)`
No of molecules in unit cell `=519.6/259.8=2`