Question

Halogenated compounds are particularly easy to identify by their mass spectra because chlorine and bromine occur naturally as mixtures of two abundant isotopes. chlorine occurs as 35cl (75.8%) and 37cl (24.2%); bromine occurs as 79br (50.7%) and 81br (49.3%); boron compounds also stand out owing to the two isotopes 10b (19.9%) and 11b (80.1%). for the compound chlorocyclohexane, c6h11cl: at what masses do the molecular ions occur? (list in order of increasing mass separated by commas,
e.g. 120,122.) what are the percentages of each molecular ion? (list to nearest 1% in order of increasing mass separated by commas,
e.g. 55,45.)

Answer 1

Chlorocyclohexane has the molecular formula of C₆H₁₁Cl. We can use simplified atomic masses for each atom to find the total molecular weight.

6C = 6 x 12 g/mol = 72 g/mol

11H = 11 x 1 g/mol = 11 g/mol

1Cl = 1 x 35 g/mol = 35 g/mol and 1Cl = 1 x 37 g/mol = 37 g/mol.

We can add the masses of each atom to get two different molar masses, one for each chlorine isotope.

72 + 11 + 35 = 118 g/mol with ³⁵Cl.

72 + 11 + 37 = 120 g/mol with ³⁷Cl.

The two molecular ions will therefore be 118, 120.

The relative percentages of each molecular ion will be solely dependent on the ratio of chlorine isotopes which we are told is roughly 76:24. Therefore, the ratio of molecular ions is:

118, 76 %.
120, 24 %.