Lab Report Finding Isotopic Abundances Using GC MS PDF

Preview

Summary

Download Lab Report Finding Isotopic Abundances Using GC MS PDF

Description

Finding Isotopic Abundances Using GC/MS By: Jonathan Pilafas (performed on 9/24/2018)

Purpose: The objective of this lab is to determine the isotopic abundances found in isotopes of chlorobenzene and bromobenzene using a gas chromatograph and a mass spectrometer.

References: 1. “Laboratory Experiments for Chemistry 121” - Harper College 2. http://dept.harpercollege.edu/chemistry/msds1/chlorobenzene.pdf 3. http://dept.harpercollege.edu/chemistry/msds1/Bromobenzene%20ScienceLab.pdf 4. http://dept.harpercollege.edu/chemistry/msds1/Hexane.pdf 5. “Mass Spectronomy Pre-Lab Assignment” - Harper College 6. “Inorganic Nomenclature” - Harper College 7. “GCMS of Chlorobenzene and Bromobenzene” - Harper College

Reagents:

Name

Molecular Weight

Boiling Point

Chlorobenzene

112.56 g/mole

132 °C

Bromobenzene

157.02 g/mole

156.2 °C

Hexane

86.18 g/mole

69 °C

Safety: Chlorobenzene - Health 2, Fire 3, Reactivity 0 Bromobenzene - Health 2, Fire 2, Reactivity 0 Hexane - Health 2, Fire 3, Reactivity 0

Instruments: -

Gas Chromatograph: Hewlett Packard GC System 6890 Series

-

Mass Spectrometer: Hewlett Packard Mass Selective Detector 5973

Methods: To begin the experiment, ensure that the chlorobenzene and bromobenzene are prepared to be tested with. This differs from the procedure in my lab notebook, which says to inject 0.2 µL mixture of chlorobenzene and bromobenzene in solvent and hexane in GC/MS. This step was already completed at the start of this lab. Obtain and analyze the mass spectrometer of chlorobenzene and bromobenzene by printing the data obtained by the GC/MS. This differs from the directions in my lab notebook, since it does not say to print the recorded data, as well as specifying that that is what needed to be analyzed. Determine the isotopic abundances of the isotopes of chlorine and bromine from their respective spectrometer. Glassware: -

10 mL volumetric pipet

-

50 mL beaker

Data: SEE NEXT PAGE FOR GRAPHS Data

Calculations:

Isotopic Abundance of Cl-35: 20.1cm Cl-35: (20.1 cm) / (20.1cm + 7.5 cm) = 72.82% Isotopic Abundance of Cl-37: 7.5 cm Cl-37: (7.5 cm) / (20.1 cm + 7.5 cm) = 27.17% Isotopic Abundance of Br-79: 12.2 cm Br-79: (12.2 cm) / (12.2 cm + 12.0 cm) = 50.41% Isotopic Abundance of Br-81: 12.0 cm Br-81: (12.0 cm) / (12.2 cm + 12.0 cm) = 49.59%

Calculated and Theoretical Isotopic Abundances

Isotope

Calculated Isotopic Abundances (experimental)

Theoretical Isotopic Abundances (from CRC)

Cl-35

72.82%

75.77%

Cl-37

27.17%

24.23%

Br-79

50.41%

50.69%

Br-81

49.59%

49.31%

Atomic Mass of Chlorine

Isotope

Isotopic Mass (amu)

Isotopic Abundance (experimental) in fractional form

Mass (amu)

Cl-35

34.968852

0.7577

26.50

Cl-37

36.965903

0.2423

8.957

Average mass (amu)

35.46

Atomic Mass of Bromine

Isotope

Isotopic Mass (amu)

Isotopic Abundance (experimental) in fractional form

Mass (amu)

Br-79

78.918336

0.5041

39.78

Br-81

80.916289

0.4959

40.13

Average mass (amu)

79.91

Discussion: The major isotope of chlorine is Cl-35 with a 72.82% calculated isotopic abundance, and the major isotope of bromine is Br-79 with a 50.41% calculated isotopic abundance. Although it is very close, neither of these calculated isotopic abundances agree with the isotopic abundances found in the CRC. The isotopic abundance for Cl-35, according to the CRC, is 75.77%. Also, the isotopic abundance for Br-79, according to the CRC, is 50.69%. The accuracy of both of the calculated isotopic abundances were both strong overall, since the calculated value for Cl-35 was off by about 4%, and the calculated value for Br-79 was off by just 1%.

The numbers I calculated in order to find the average atomic mass of chlorine agree with the numbers on the periodic table tremendously. The average mass for chlorine was calculated to be 35.46, which was only 0.0003% off when compared with the average mass for chlorine on the periodic table at 35.45. The average mass for bromine was calculated to be 79.91, which was only 0.0001% off when compared with the average mass for bromine on the periodic table at 79.90. The molar mass of chlorobenzene was found to be 112 amu, and the molar mass of bromobenzene was found to be 156 amu. These molar mass peaks were found using the mass spectrum. This device was used to record all of the potential molar masses of these substances, determine the most commonly occurring atomic mass, and list the exact atomic mass on the top of the substances’ tested molar masses. The fragment of the molecule that gives a peak of 77 m/z is benzene. Isotope

Protons

Neutrons

Electrons

Cl-35

17

18

17

Cl-37

17

20

19

Br-79

35

44

34

Br-81

35

46

36

Given the calculated natural abundance of chlorine and bromine, what I think the mass spectrum of each of the pure elements would look is listed on the following page:...