Because the mass of the reactants equal to the mass of the product of a reaction, the number of moles will be conserved.

Question

-I think this is sometime true but I don't know a way to prove that it sometime true. Anyone got an idea ? Thanks

Answer 1

It seldom is true.
examples:
Fe+S=FeS (two moles=1?)
2Fe+O2> 2FeO (three moles=2?)
N2 + O2 > 2NO (two moles=2)