element combine with another element to form a compound. C2H6, 2 atoms of carbon combine with 6 atoms of hydrogen to form ethane. Mn(Cr2O7)2, one Mn combines with 4 Cr and 14 O to form manganese(II) dichromate. If we know the formula of a compound, determining the molar mass is simple. Since we know the atomic masses of the elements, we can sum the atomic. masses to obtain the molecular masses. 1 atom of C2H6 contains 2 atoms of C 12.01 amu = 24.02.