A metal crystallizes into two cubic phases, face-centred cubic and body-centred cubic, which have unit cell lengths `3.5` and `3.0 A`, respectively. Calculate the ration of densities of fcc and bcc.

Explanation

Density `(rho) = (Z_(eff) xx Mw)/(a^(3) xx N_(A)`
For fcc, `Z_(eff) = 4`
`rho_(fc c) = (4 xx Mw)/(3.5 xx 10^(-8))^(3) xx N_(A)`
For fcc, `Z_(eff) = 2`
`rho_(fc c) = (2 xx Mw)/(3 xx 10^(-8))^(3) xx N_(A)`
`(rho_(fc c)/(rho_(fcc) = (4 xx (3)^(3))/((3.5)^(3) xx 2 = 1.26`