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Ms Ans

This document discusses mass spectrometry and provides 10 exercises analyzing mass spectral data to determine molecular formulas and structures of unknown substances. The exercises involve interpreting molecular ion peaks, isotope peaks, and fragment peaks to deduce the number of carbons, hydrogens, and other elements in molecular formulas. Structural information can also be inferred from characteristic fragment peaks.

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Ahmed Zaky
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805 views5 pages

Ms Ans

This document discusses mass spectrometry and provides 10 exercises analyzing mass spectral data to determine molecular formulas and structures of unknown substances. The exercises involve interpreting molecular ion peaks, isotope peaks, and fragment peaks to deduce the number of carbons, hydrogens, and other elements in molecular formulas. Structural information can also be inferred from characteristic fragment peaks.

Uploaded by

Ahmed Zaky
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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MASS SPECTROMETRY (MS)

Exercise 1:
Determine the degree of unsaturation (IHD) for the hydrocarbons with the following molecular
formulas:

(a) C10H16 HDI = 3

(b) C7H7NO HDI = 5

(c) C8H9ClO HDI = 4

Exercise 2:
An unknown substance shows a molecular ion peak at m/z = 170 with a relative intensity of 100.
The M + 1 peak has an intensity of 13.2, and the M + 2 peak has an intensity of 1.00. What is the
molecular formula of the unknown?

M+ at m/z = 170 implies an even # of nitrogen


M+ peak it is the base peak, no recalculation necessary
M + 1 peak 13.2 / 1.1 = 12; 12 carbons
M + 2 peak 1.00% of M+; no S, Cl, Br

170 – (12)(12) = 26 C12H26 is possible


cannot have two N MM exceeded
M + 2 peak its presence might indicate oxygen
170 – (12)(12) – 16 = 10 C12H10O

answer: C12H10O (or C12H26)

Exercise 3:
An unknown hydrocarbon has a molecular ion peak at m/z = 84, with a relative intensity of 31.3.
The M + 1 peak has a relative intensity of 2.06, and the M + 2 peak has a relative intensity of 0.08. What
is the molecular formula for this substance?

M+ at m/z = 84 implies an even # of nitrogen


M+ peak it is not the base peak, recalculation necessary
M+ 31.3 100%
M+1 2.06 6.58% 6.58 / 1.1 = 5.98; 6 carbons
M+2 0.08 0.25% no S, Cl, Br
84 – (6)(12) = 12

answer: C6H12
Page | 1
Exercise 4:
An unknown substance has a molecular ion peak at m/z = 107, with a relative intensity of 100. The
relative intensity of the M + 1 peak is 8.00, and the relative intensity of the M + 2 peak is 0.30. What is
the molecular formula for this unknown?

M+ at m/z = 107 implies an odd # of nitrogen


M+ peak it is the base peak, no recalculation necessary
M + 1 peak 8.00 / 1.1 = 7.3; 7 carbons
M + 2 peak 0.30%; no S, Cl, Br
107 – (1)(14) – (7)(12) = 9

answer: C7H9N

Exercise 5:
The mass spectral data of an unknown liquid are given below. What is the molecular formula of this
unknown?

M+ at m/z = 78 implies an even # of nitrogen


M+ peak it is not the base peak, recalculation necessary
M+ 23.6 100%
M+1 0.79 3.35% 3.35 / 1.1 = 3.05; 3 carbons
M+2 7.55 32% Cl is present
78 – (3)(12) – 35 = 7

answer: C3H7Cl

Exercise 6:
Propose a molecular formula for each mass spectrum:

Page | 2
Exercise 7:
Look at the mass spectrum of 2-methylhexane. What is the m/z value of the M+ peak and of the base
peak? Give possible structures of the fragments giving rise to the large peaks at m/z = 85, 57, and 43.

M+ peak m/z = 100


base peak m/z = 43

Page | 3
Exercise 8:
Give the structure of the molecule that gives the following mass spectrum.

Exercise 9:
The mass spectra of heptane and 2,2,3-trimethylbutane are given below. Which spectrum represents
which compound? Explain.

Page | 4
Exercise 10:
Propose a structure for the compound having the following mass spectrum:

(a)

(b)

Page | 5

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