Module 1 - Stoich and review

Module 1 - Stoich and review - CHEM 120 1.1 Stoichiometry...

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CHEM 120 Module 1: Stoichiometry Review page 1 of 10 1.1 Stoichiometry Stoichiometry is the quantitative study of the composition of compounds and mixtures, and of the amounts of reactants or products involved in a chemical reaction. compound mixture In short, stoichiometry is the study and use of quantitative relationships involving amounts of matter. In the lab, we are (usually) restricted to using samples of matter that are large enough to see and manipulate using laboratory equipment. Such samples typically contain uncountable numbers of atoms or molecules. Consequently, counting atoms and molecules directly is simply out of the question. We are forced to deduce the number of atoms or molecules indirectly from some macroscopic measure of amount (e.g. mass). 1.2 The Mole The SI unit for the amount of substance is the mole . The abbreviation for the mole is mol . Why is the mole defined as such a funny number? The formal definition of the mole is that it is the number of atoms in exactly 12 grams of 12 C. (Note: 12 C is the most abundant isotope of carbon.) Scientists have measured the mass of one 12 C, using a mass spectrometer, and its mass is 1.9927×10 23 g. Therefore: 12 12 23 12 -23 12 one C atom 12 g C × = 6.022×10 C atoms 1.9927×10 g C composed of two or more elements; has a fixed composition e.g. water is always 89% oxygen by mass and 11% hydrogen e.g. ethanol is always 52% carbon, 13% hydrogen and 35% oxygen composed of two or more compounds; has a variable composition e.g. water and ethanol can be mixed in any proportion 1 mol = 6.022×10 23 particles Related units: 1 mmol = 10 3 mol 1 μ mol = 10 6 mol a micromole
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CHEM 120 Module 1: Stoichiometry Review page 2 of 10 Other important quantities atomic mass unit = (amu or u) average atomic mass = Avogadro’s number ( N A ): N A = 6.022×10 23 mol 1 Molar mass = (g mol 1 ) The masses given in a periodic table have two equivalent interpretations. For example, consider the entry for carbon. The molar mass allows us to calculate the number of moles, n , in a weighed sample!! 6 C 12.011 1 12 of the mass of one 12 C atom Therefore, a single atom of 12 C has a mass of 12 u. “u” is the preferred abbreviation weighted average of the atomic masses of the various isotopes For example, the two most abundant isotopes of carbon are 12 C and 13 C with masses of 12.0000 u and 13.0035 u, respectively, and natural abundances of 98.89% and 1.11%, respectively. The average atomic mass is (0.9889)(12.0000 u) + (0.0111)(13.0035) = 12.01 u. The average atomic mass is the mass reported on a periodic table. Note that N A has units of “mol 1 ”. mass (in grams) of one mole atomic number (Z) = # protons in the nucleus average atomic mass of carbon (in u) or molar mass of carbon (in g mol 1 ) The molar masses of the elements can be combined appropriately to obtain the molar mass of a compound. For example, the molar mass of C is 12.01 g mol 1 and the molar mass of H is 1.008 g mol 1 .
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This note was uploaded on 06/07/2011 for the course CHEM 120 taught by Professor Gilbert during the Fall '08 term at Waterloo.

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Module 1 - Stoich and review - CHEM 120 1.1 Stoichiometry...

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