In Mass Spectrometry (MS) the molar mass of compounds can be determined. The so-

Question

In Mass Spectrometry (MS) the molar mass of compounds can be determined. The so- called "molecular ion" corresponds to the molecular mass when Electron Impact Mass Spectrometry (EIMS. one of several common MS techniques) is used. It is important to realize, though, that the mass detected by MS depends on the isotopes present in the molecule. The questions below try illustrate the impact of isotope distribution in a molecule. What is the molar mass of the compound shown below? What is the molar mass if only 35^Cl isotopes are present in the molecule? What is the molar mass if 90 % of all carbon atoms consist of l2^C, 10 % of 13^C isotopes?

Explanation / Answer

(a)

Chemical Formula: C16H15ClO

Molecular Weight: (12 x 16) + (1 x 15) + 35 + 16 = 258 g/mol

(b)

There are two principal stable isotopes, 35Cl (75.78%) and 37Cl (24.22%), giving chlorine a relative atomic mass of 35.453. The longest-lived radioactive isotope is 36Cl, which has a half-life of 301,000 years.

Chemical Formula: C16H15ClO

Molecular Weight: 258 g/mol ( The molar mass calculated with mass of Cl as 35).

(c)

The mass of 12C is, of course, 12 amu. 13C is 1.0836129 times heavier than 12C; so, the mass of 13C is 13.003355

Average mass C = (0.90 x 12) amu + (0.10 x 13.00335) amu

= 12.10

Now,

Chemical Formula: C16H15ClO

Molecular Weight: = 12.10 x 16 + 1 x 15 + 35 + 16 = 259.6 g/mol

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