Chem 010 Lab 11 Avogadros Number

Chem 010 Lab 11 Avogadros Number - A Fatty Acid Monolayer:...

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A Fatty Acid Monolayer: Determination of Avogadro’s Number CHEM 010 – Laboratory 11 Purpose In this laboratory experiment you will estimate Avogadro’s number by making some simple measurements of a fatty acid monolayer formed on the surface of water. Background Avogadro’s number is named after the Italian physicist and lawyer Almedo Avogadro, who, though he conceived the basic idea, never knew the actual value. Avogadro’s number, N A , is defined as the number of atoms in exactly 12 g of carbon-12 and is currently accepted to be 6.0221367 x 10 23 . The quantity of a substance which contains Avogadro’s number of particles (atoms, molecules, formula units, etc .) is known as a mole of the substance. Since it is clearly not possible, even with the fastest computer, to count all of the atoms in a mole, how has Avogadro’s number been determined? Over the years there have been many different experimental methods developed to measure this value. One technique, a variation of which we will be using in this laboratory experiment, counts the number of molecules present in a sample of compound with a known mass. Multiplication of the number of molecules per gram by the molar mass of the compound yields Avogadro’s number. In this experiment we will determine the number of molecules required to form a film of an oily substance, stearic acid, one molecule thick (a monolayer) on a water surface. Stearic acid (C 17 H 35 COOH) is a member of a class of substances known as fatty acids due to their presence in many natural fats. Fatty acids are long “hydrocarbon chains” which terminate in a carboxylic acid group (-COOH). The polar carboxylic acid group is strongly attracted to water (hydrophilic or “water loving”) through hydrogen bonding interactions. The long hydrocarbon chains are greasy and are repelled by water (hydrophobic or “water hating”). Due to the presence of the long hydrophobic hydrocarbon chains, fatty acids are generally insoluble in water. When a solution of stearic acid in a volatile hydrocarbon solvent is placed on water, the stearic acid molecules spread out on the surface of the water with the hydrophilic “heads” (-COOH) stuck into the water and the hydrophobic “tails” projecting out into the space above the water. The water insoluble hydrocarbon solvent evaporates, leaving behind a monolayer of stearic acid. Additional stearic acid solution will cause the monolayer to spread until the entire water surface is covered. Subsequent solution added will float on top of the existing monolayer to form a lens-shaped droplet that is readily visible. The exact number of drops of stearic acid solution required to form the monolayer is readily apparent from the number of drops required to see the lensing effect.
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This note was uploaded on 12/11/2010 for the course CHEM 10 taught by Professor Annekelley during the Fall '10 term at UC Merced.

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Chem 010 Lab 11 Avogadros Number - A Fatty Acid Monolayer:...

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