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Molecules 1/30/2009
Class and Moles
CHEM 107 Texas A&M University
What's a molecule?
Again, Again from our glossary: "The smallest particle of an element or compound that can exist and retain the chemical properties of that element or compound."
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Molecules
Molecular formula gives composition: number of atoms of each element present Molecular mass = sum of masses of atoms
Representing Molecules
Names: ethanol, ethyl alcohol Formulas C2H6O, C2H5OH, CH3CH2OH Structural drawings g Models
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Ways to depict molecules
ethanol C 2H 6O CH3CH2OH dimethyl ether C 2H 6O CH3OCH3
Models of molecules
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Line Drawings
H H C H H C H O H OH
Shorthand notation y g Carbons & hydrogens on carbons not shown Each line is a bond Each carbon has 4 bonds
Example: Line structure / formula
N O C O
This compound has potential as a medium for holographic data storage.
Find formula
N
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Example
N O C O
Formula is C15H20N2O2
N
Chemistry: 3 "Representations"
Macroscopic or bulk: observations Symbolic: written description Microscopic/molecular: visualize atoms & molecules One big "trick" to chemistry is seeing connections between these views
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Molecules & Moles
Number of molecules in any macroscopic sample is huge Use moles as a unit of quantity 1 mole = 6.0221415 x 1023 This is an experimentally measured number ("Avogadro's Number")
Avogadro's Number
Defined by setting 1 mole 12C = 12 g (exactly) Definition of the amu is 1 atom 12C = 12 amu (exactly) Mass of a single atom or molecule in g amu is numerically equal to mass of one mole in grams
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Mass, moles, # of molecules
Converting between these is just changing units. Use atomic & molecular weights, densities, etc. as needed.
Simple Mole Calculation
A 12-inch cast iron skillet weighs about 12 inch 8.4 pounds. How many moles of iron is that? How many iron atoms?
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Mole Problem
8.4 8 4 lbs Fe = ? Fe Moles What do we know? atomic wt. of Fe = 55.85, so: 1 mole Fe = 55.85 g Fe 454 g = 1 lb Now just convert units! lbs g moles
Mole Problem
8.4 8 4 lbs Fe = ? Moles Fe 1 mole Fe = 55.85 g Fe 1 pound = 454 g 454 g 1 mole Fe 8.4 lbs Fe x x 1 lb 55.85 55 85 g
= 68.3 mol Fe
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Molar mass of molecules
Conservation of mass means that: Molar mass of a molecule is the sum of the molar masses of the atoms in the molecule. Water: H2O 2 H's + 1 O 2(1.0079 g/mol) + 1(15.9994 g/mol) = 18.0152 g/mol
Example from last time
N O C O
Formula is C15H20N2O2 Molar mass?
N
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1/30/2009
Example from last time
N O C O
N
Formula is C15H20N2O2 Atomic wts: C=12, H=1, N=14, O = 16 Molar 15 x 12 = 180 mass 20 x 1 = 20 2 x 14 = 28 2 x 16 = 32 260
Formulas & % composition
Given a chemical formula it's easy to formula, it s find mass %' s Find molar mass and the mass from each element As an example, calculate mass % of p , each element in the previous molecule
69% C, 7.7% H, 11% N, 12% O
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Composition & formulas
Reverse process is useful in analyzing unknowns Instruments can tell us the elemental composition of a substance ("elemental analysis") We can convert this info into a molecular formula
Composition & formulas: Example
Nicotine contains 74 0% C 8 65% H 74.0% C, 8.65% H, and 17.35% N. If the molar mass of nicotine is 162, what is the chemical formula of nicotine? Atomic weights for C, H, and N are 12, 1, and 14.
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Nicotine structure
N N
Nicotine C10H14N2
% Composition & Formula
Nicotine contains 74 0% C 8 65% H 74.0% C, 8.65% H, and 17.35% N. (If the molar mass of nicotine is not known), What is the chemical formula of nicotine? Atomic weights for C, H, and N are 12, 1, and 14.
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Empirical vs. Molecular formulas
Nicotine shows the difference between these ideas Empirical formula: simplest possible formula with correct ratios of atoms Molecular formula: formula showing actual composition of a molecule
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