(1) According to the relative molecular mass, it is the sum of the relative atomic masses of the atoms that make up the molecule; The relative molecular weight of benzoyl oxide C 14H 10O4 is:12×14+16× 4 = 242; So the answer is: 242;

② According to the mass fraction of an element in a substance = the relative atomic mass of the element × the number of atoms × the relative molecular mass of the substance × 100%.

Mass fraction of carbon in benzoyl peroxide =168242×100% = 69.4%?

So the answer is: 69.4%.