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Chapter 6 Chemistry Notes - Why Is Knowledge of Composition...

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9/23/2013 1 Chapter 6 - Chemical Composition (The Mole) Most matter is a mixture To define the mixture we must measure composition What are ways to measure amounts? Mass and Counting Conversions From submicroscopic macroscopic The MOLE as solution to many chemical problems Why Is Knowledge of Composition Important? Nearly all matter is either a chemical or physical combination of elements. To understand matter, (1) we need to know its composition and (2) how it is put together. What is one of my favorite mixture of elements? http://commons.wikimedia.org/wiki/File:French_chocolate_cake.jpg Measuring Out Matter (mmmmm) Cake: 2 c unbleached all purpose flour 1 3/4 c sugar, 3/4 c cocoa, 2 t baking soda, 1/4 t salt, 1 c water , 3/4 c buttermilk, 3/4 c vegetable oil 3 large eggs Chocolate ganache and raspberry topping: 18 ounces bittersweet chocolate (do not exceed 61% cacao), 2 1/4 c heavy whipping cream, ¾ lbs fresh raspberries , 6 T raspberry jam How are amounts measured in this recipe? http://www.epicurious.com/recipes/food/views/Chocolate-Raspberry-Layer-Cake-359411#ixzz101Kc6DNN Measuring amount In the kitchen (or in te lab), how do we measure amounts? volume, counting, and mass (or weight). Volume is convenient but is it reliable? What often happens to volume when you heat or cool a substance? ______________________________ Quick experiment: If you add 75.0 mL of ethanol and 25.0 mL of water, what is the resulting volume? What do you expect? ________mL What do we find? _______mL Conclusion: our best measures of amount are counting and mass. (Volume is sometime convenient but must be used carefully. If needed density can be used to convert the volume to mass, and always remember volume is not necessarily conserved) . We already know mass and counting are related. Macroscopic Sub-Microscopic Symbolically Models/ Word/ Math equations Law of Constant Composition Macroscopic mass Atomic level mass If we know the atomic mass of H (1 amu) and O (16 amu), we can find out the number of each atom. (recall: 1 amu = 1.6x10 -24 g) H’s=1g/1.6x10 -24 g=6x10 23 O’s=8g/(16*1.6x10 -24 g)=3x10 23 **Atom ratio: 2 H per 1 O! 1g H 8 g Oxygen http://commons.wikimedia.org/wiki/File:Water_molecule.png Macroscopic Symbolic Molecular Propane Ethylene glycol At the sub microscopic level the relationship is simple: Chemical formula Formula Mass C 2 H 6 O 2 FW=62.08 amu C 3 H 8 FW=44.11 amu Formula Mass – Sum atomic masses of all atoms in formula. (masses as amu in periodic table) H 2 O Formula Mass (FM) = 2 (1.01 amu) + 1 (16.00 amu) = 18.02 amu Wouldn’t it be nice if it were easy to relate # of atoms to mass for macroscopic samples too?
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9/23/2013 2 Example: WW-I Chemical Weapon Phosgene gas (COCl 2 ) was used as a chemical weapon during WW-I but now is used as a building block for pharmaceuticals.
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