Lecture notes Chap.3 - GENERAL CHEMISTRY CHAPTER 3 MASS...

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GENERAL CHEMISTRY PAGE 1 OF 18 The sample (containing the atoms whose mass is to be measured) is injected into the instrument and vaporized Vaporized atoms are ionized by an electron beam The electrons in the beam collide with the vaporized atoms, removing electrons from the atoms and creating positively charged ions (cations) Charged plates with slits in them accelerate the positively charged ions into a magnetic field, which deflects them The amount of deflection depends on the mass of the ions Lighter ions are deflected more than heavier ones The ions strike a detector and produce an electrical signal that is recorded The result is the separation of the atoms in the sample according to their mass = mass spectrum The x -axis gives the mass of the isotope that was ionized The intensity (height of the peak) on the y -axis gives the relative abundance of that isotope Atomic Masses By observing the proportions in which elements combine to form various compounds, 19 th century chemists calculated relative atomic masses Modern system of atomic masses instituted in 1961, is based on 12 C (carbon 12) as the standard In this system, 12 C is assigned a mass of exactly 12 atomic mass units (amu) Masses of all other atoms are given relative to this standard Recall: 1 12 x (12 amu) = 1 amu NOTE: 12 amu is an exact number by definition Example : Experiments show that a hydrogen atom is only 8.400% as massive as the 12 C atom The atomic mass of hydrogen = 8.400 100 x (12.001 amu) = 1.008 amu Mass Spectrometry The most accurate method currently available for comparing the masses of atoms is the mass spectrometer Measures masses and abundances of isotopes
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GENERAL CHEMISTRY PAGE 2 OF 18 Uses of Mass Spec Mass spectrometry can also be used on molecules Because molecules often break apart during ionization, the mass spectrum of a molecule usually contains many peaks representing the masses of different parts of the molecule, as well as a peak representing the mass of a molecule as a whole The fragments that form upon ionization and the corresponding peaks in the mass spectrum are specific to the molecule, so that the mass spectrum is = molecular fingerprint Used to identify an unknown molecules Used to determine how much a molecule is present in a sample Proteins and nucleic acids A tumor can be analyzed to determine whether it contains specific proteins associated with cancer Atomic Mass of an Element The atomic masses listed on the periodic table for each element are average atomic masses and include the abundance of each isotope Atomic Mass = Σ (fractional abundance of isotope) n x (mass of isotope) n Example : If chlorine is 75.77% C1-35 with a mass of 34.97 amu and the rest is 24.23% C1-37 with a mass of 36.97 amu, what is chlorines atomic mass? Atomic mass Cl
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This note was uploaded on 11/02/2010 for the course CHEMISTRY 22321 taught by Professor Wilcox during the Spring '10 term at Fullerton College.

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Lecture notes Chap.3 - GENERAL CHEMISTRY CHAPTER 3 MASS...

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