CsBr crystallises in a body-centred cubic (bcc) lattice. The unit cell lengt is 436.6pm. Given that the atomic mass of Cs= 133u and that of Br=80u. What will be the density of CsBr? a) 0.425g/cm^3 b) 8.25g/cm^3 c) 42.5g/cm^3 d) 4.25g/cm^3
Calculate the mass of one atom of CsBr from the atomic masses of Cs and Br
133u + 80u = 213u
first convert pm to cm.
Calculate the volume of unit cell
Now you must know
density = mass/volume
we have the mass already but convert amu to grams
213amu = 3.5369e-22
= 4.25g/cm3 (three sig figs)
Thus option d is correct
Another way is
Where a^3 = atomic radius^3 which is = volume
You would see that Z for bbc's are usually 2 but for this bbc Z = 1
because 1 bbc has 2atoms which here means 1atom of Cs and 1atom of Br giving a total of 1molecule and Z is the no. of molecule per bbc. The bbc is usually 2 because bbc's are of atoms and elements have 1 atom which make up a molecule .