To determine the molecular formula of the unknown compound, we can start by examining the molecular ion peak at m/z 120. This peak represents the mass of the entire molecule. We can then look at the fragment ions to help deduce the structure of the compound.Fragment ions at m/z 91, 77, 63, and 45 indicate that the molecule is breaking apart into smaller pieces. These fragment ions can give us clues about the structure and elements present in the compound.First, let's consider the most common elements found in organic compounds: carbon C , hydrogen H , oxygen O , and nitrogen N . The atomic masses of these elements are approximately 12, 1, 16, and 14, respectively.Now, let's try to find a molecular formula that adds up to a mass of 120:C6H12O2: 6 x 12 + 12 x 1 + 2 x 16 = 72 + 12 + 32 = 116 close, but not quite 120 C5H8O3: 5 x 12 + 8 x 1 + 3 x 16 = 60 + 8 + 48 = 116 again, close but not 120 C5H12O2N: 5 x 12 + 12 x 1 + 2 x 16 + 1 x 14 = 60 + 12 + 32 + 14 = 118 still not 120 C6H12O1N1: 6 x 12 + 12 x 1 + 1 x 16 + 1 x 14 = 72 + 12 + 16 + 14 = 114 not 120 C5H10O2N1: 5 x 12 + 10 x 1 + 2 x 16 + 1 x 14 = 60 + 10 + 32 + 14 = 116 not 120 C6H9NO2: 6 x 12 + 9 x 1 + 1 x 14 + 2 x 16 = 72 + 9 + 14 + 32 = 127 not 120 C5H8N2O2: 5 x 12 + 8 x 1 + 2 x 14 + 2 x 16 = 60 + 8 + 28 + 32 = 128 not 120 C6H12O2N: 6 x 12 + 12 x 1 + 2 x 16 + 1 x 14 = 72 + 12 + 32 + 14 = 130 not 120 After trying several combinations, we find that the molecular formula C5H8N2O2 has a mass of 128, which is not equal to the molecular ion peak at m/z 120. Therefore, we need to consider other elements or combinations of elements to find the correct molecular formula.One possibility is that the compound contains a halogen, such as chlorine Cl or bromine Br , which have atomic masses of approximately 35.5 and 79.9, respectively. Let's try a molecular formula containing chlorine:C4H8O1N1Cl1: 4 x 12 + 8 x 1 + 1 x 16 + 1 x 14 + 1 x 35.5 = 48 + 8 + 16 + 14 + 35.5 = 121.5 not 120 It seems that including chlorine in the molecular formula does not result in a mass of 120 either. Therefore, it is difficult to determine the exact molecular formula of the unknown compound based solely on the given information. Additional data, such as infrared IR or nuclear magnetic resonance NMR spectroscopy, would be necessary to confirm the molecular formula and structure of the compound.