Answer
$0.533\ g/cm^3$
Work Step by Step
For a body-centered cubic arrangement there are $8 \times 1/8$ atoms in the corners and one in the center, so 2 atoms per unit cell:
Mass: $2\times 6.94\ g/mol\div 6.022\times 10^{23}\ atoms/mol=2.31\times10^{-23}\ g/cell$
Volume: $(351\times 10^{-10}\ cm)^3=4.32\times10^{-23}\ cm^3/cell$
Density: $2.31\times10^{-23}\ g/cell\div 4.32\times10^{-23}\ cm^3/cell=0.533\ g/cm^3$
