1) Ta có : 56.2 + 96.x=400
  => 96.x = 288
  => x = 3 
CTHH: Fe2(SO4)3
2) Ta có: 56.x + 16.3 = 160
  => 56.x = 112 
  => x = 2
CTHH: Fe2O3
3) Ta có: 27.2 + 96.x = 342
  => 96.x = 288
  =>x = 3
CTHH: Al2(SO4)3
4) Ta có: 39.2 + 96.x = 174  
  => 96.x =96
  => x = 1
CTHH: K2SO4
5) Ta có: 40.x + 95.2 = 310
  => 40.x = 120
  => x = 3
CTHH: Ca3(PO4)2