Question
Sat July 02, 2011 By: Akshaya Parthasarathy

calculate the avagadro number if iron crystallises as fcc and its density is 2.8g/cm^3 . if the volume of unit cell is 56.38 * 10^-24 cm^3/unit cell ( atomis mass = 56 )

Expert Reply
Mon July 04, 2011
1.Density= 2.8 g/cm3.
  As it crystallises in fcc lattice ,
  i.e.there are '8' Iron atoms at the corners and there contribution is 1/8.
      and 6 at 6 faces of the cube
     and atoms at the faces contribute=1/2.
    Therefore, total no. of Iron atoms per cube,(Z)=(8*1/8+6*1/2)=(1+3)=4 atoms.
 
Density=mass/volume
volume=(56.38*10-24)cm3/unit cell.
mass of single = Z*(56)/NA,  where NA=Avogadro's number.
Therefore, density*volume=Z*(55.84)/NA.
2.8*56.84=4*56/NA.
NA=4*56/2.8*56.84.
Home Work Help