Answer :
Fe atomic mass = 55.86
* 1 = 55.86
Cr atomic mass = 52
*2 = 104
O atomic mass = 16
* 4 = 64
Total molecular mass = 223.86
Percent by mass Fe = (55.86/223.86)*100 = 24.95%
Percent by mass Cr = 46.45%
Percent by mass O = 28.59%
The mass of each elements in chromite is as below
Fe =24.9 %
Cr =46.5%
O = 28.6%
calculation
percent mass = atomic mass of element / atomic mass of FeCr2O4 x 100
from periodic table the atomic mass for Fe = 55.85 g/mol,
Cr =52 g/mol since they are two atoms in FeCr2O4 = 2 x 52 = 104 g/mol
for O = 16 since they are two atoms in FeCr2O4 = 16 x4 = 64 g/mol
Atomic mass for FeCr2O4 is = 55.85 +104+ 64 = 223.85 g/mol
Percent mass for Fe =55.85 /223.85 x100 = 24.9 %
for Cr = 104 / 223.85 x 100 = 46.5%
For 0 = 64/223.85 x 100 = 28.6%