Studio_2_Avogadro_FINAL_F2006 - Chem 25 Studio#2 NAME...

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Chem 25: Studio #2 Determination of Avogadro’s Number NAME:___________________________ STUDIO:___________________________ Determination of Avogadro’s Number from a Stearic Acid Monolayer Although we honor him by the name of the quantity, Amadeo Avogadro (1776- 1856) did not suggest the concept of the mole and never determined how many particles constituted that unit. A few years after Avogadro’s death, a German scientist, Joseph Loschmidt, estimated the size of a molecule which made it possible for others to estimate the number of molecules in a volume of gas. Because Avogadro hypothesized that equal volumes of gases at the same temperature and pressure contained equal numbers of particles, Loschmidt’s estimate is only one step away from determining the number of particles in a mole. The French scientist Jean Perrin is credited with determining the first value of Avogadro’s number in 1908; he got values in the range of 5.4 to 6.0 x 10 23 . Once the American physicist Robert Milliken determined the charge on an electron in 1915, a more accurate value was obtained by dividing the Faraday constant (the charge on a mole of electrons) by the charge on a single electron. Finally, modern sophisticated equipment has yielded the most accurate value of Avogadro’s number to date: N A = 6.02214199 x 10 23 We’ll typically use the number expressed to three digits, 6.02 x 10 23 . A mole of anything contains Avogadro’s number of particles. For chemical entities – electrons, atoms, ions, molecules – a mole of material is an amount that contains the same number of particles as there are C-12 atoms in 12.00000 g of C-12. Samples of monoatomic elements that have masses corresponding to their atomic mass expressed in grams contain one mole of atoms. Thus 26.98 g of Al contain 6.02 x 10 23 atoms of aluminum and hence one mole of aluminum. One mole of items may consist of: atoms – if we are dealing with atoms (39.95 g Ar/mole Ar atoms; 35.45 g Cl/mole Cl atoms) molecules – if we are dealing with compounds which exist as discrete molecules (70.90 g of Cl 2 / mole of Cl 2 molecules; 342.30 g of C 12 H 22 O 11 / mole of sucrose molecules) formula units – if we are dealing with a compound that does not form discrete molecules (58.44 g NaCl / mole of NaCl) Avogadro’s number and the idea of a mole provide us with two of the most important conversion factors we’ll use throughout chemistry when we solve quantitative problems dealing with chemical reactions. We use conversion factors in dimensional analysis 1
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when we need to convert one quantity into another with different units. Here’s the basic idea: Conversion factor = desired units starting units and by extension: information sought = information given x desired units starting units A common use of Avogadro’s number and molar mass is to convert mass of a material into number of moles or number of constituent units: x 1 x (N A ) molar mass x molar mass x 1 N A Molar mass = # g particles N A = Avogadro’s number = 6.02 x 10
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This note was uploaded on 02/26/2008 for the course CHEM 025 taught by Professor X during the Fall '06 term at Lehigh University .

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Studio_2_Avogadro_FINAL_F2006 - Chem 25 Studio#2 NAME...

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