Question

Which law of chemical combination is illustrated by the fact that 168 g Fe reacts with 72 g H2O to from 232g Fe3 O4 and 8g H2 ?

Answer 1

At reactant side molar mass of Fe and H2O= 168+72=240g.m
At product side molar mass of Fe3O4 and H2= 232+8=240g.m
As mass constant so it is obeying
"LAW OF CONSERVATION OF MASS"

Answer 2

In this case 
168g + 72 g = 240 g LHS  Reactants 
232g + 8g = 240 g RHS Products
law of conservation of mass