Ch 3_100911 - 10/9/2011 Chapter 3 Atomic Masses(3.1) The...

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10/9/2011 1 Chapter 3 Atomic Masses(3.1) The Mole (3.2) Molar Mass (3.3) Percent Composition (3.4) Determining the Formula of a Compound (3.5) Chemical Equations (3.6) Balancing Chemical Equations (3.7) Introducing Stoichiometry (3.8) Introducing Limiting Reagents (3.9) This is the outline for the content we will cover in lecture. Please read the entire chapter. 2 A Brief History of the amu Stanislao Canizzaro (1826-1910) proposed that the H atom be used as a standard of mass and set its atomic mass at 2. Other chemists of the day wanted to use a more massive atom to reduce experimental error. Chemists eventually took the mass of naturally occurring oxygen (O) to be 16 amu. Concurrently, physicists defined the oxygen-16 isotope as 16 amu. Are these two definitions that use oxygen the same? NO . Naturally occurring oxygen is a combination of three stable isotopes, oxygen-16, oxygen-17, and oxygen-18. These two definitions resulted in conflicting values. Finally in the 1950s the carbon-12 isotope was adapted as the standard…hence 1 amu = 1/12 the mass of one 12 C atom.
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10/9/2011 2 3 Some Isotope Comparisons Element Isotope Mass of Isotope (amu) Relative Abundance Average Atomic Mass (amu) Hydrogen 2 H 2.01410 1.0079 Carbon 13 C 13.00335 12.01115 Sulfur 33 S 34 S 36 S 32.971458 33.967867 35.967080 32.064 Lithium 6 Li 6.015123 6.941 0.0156% 1.108% 0.74% 4.18% 0.0136% 7.5% Calculating Atomic Mass Let’s say we have a sample of 1000 carbon (C) atoms. Based on isotopic abundance: 989 weigh 12 amu (98.9%) 11 weigh 13 amu (1.1%) What is the average mass of a carbon atom in this sample? Atomic mass = (989 C atoms)*(12 amu) + (11 C atoms)*(13 amu) 1000 C atoms = (989 C atoms)*(12 amu) + (11 C atoms)*(13 amu) 1000 C atoms 1000 C atoms = (989/1000)*(12 amu) + (11/1000)*(13 amu) = (0.989)*(12 amu) + (0.011)*(13 amu) = 12.011 amu
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10/9/2011 3 5 Calculating Atomic Mass Let’s say we have a sample of 1000 lithium (Li) atoms. Based on isotopic abundance: 925 weigh 7 amu (92.5%) 75 weigh 6 amu (7.5%) What is the average mass of a lithium atom in this sample? Atomic mass = (0.925)*(7 amu) + (0.075)*(6 amu) = 6.9 amu 6 Don’t confuse “Atomic Mass” with the mass of one atom!! An atom can be only one isotope at a time. 12 C: Z = 12, isotopic mass = 12 amu (exactly) 13 C: Z = 13, isotopic mass = 13.003354 amu The Atomic Mass (aka Atomic Weight or Average Atomic Mass ) is the average of the atomic masses of all of the element's isotopes , weighted by isotopic abundance. Naturally occurring carbon has an atomic mass of 12.011 amu There is no carbon isotope that weighs 12.011 amu.
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10/9/2011 4 7 Determining isotopic mass using a Mass Spectrometer Stream of vaporized atoms is bombarded with high- speed electrons, which knock electrons off the gaseous atoms, turning them into cations. Gaseous cations are accelerated through magnetic field, and their paths are bent according to their mass. Counting by Weighing Chemical reactions occur at the microscopic level, between individual molecules and/or atoms.
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This note was uploaded on 01/18/2012 for the course CHEM 142B taught by Professor John during the Fall '11 term at University of Washington.

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Ch 3_100911 - 10/9/2011 Chapter 3 Atomic Masses(3.1) The...

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