Lab - Determination of Avogadro’s Number
Uploaded by erobinson on Apr 30, 2004
Determination of Avogadro’s Number
Introduction:
In this experiment we will determine Avogadro’s number by calculating the area of a one-molecule thick layer of oleic acid. Because we know the volume of one molecule, we can solve for area. Then, we use density and the molar mass of oleic acid to find Avogadro’s number.
Data:
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Discussion:
The purpose of this lab was to give us a different perspective of where Avogadro’s number comes from. We learn in a hands-on approach how it can be determined by the unique properties of oleic acid, water, and pentane.
The first step in this lab was to achieve a properly diluted solution of oleic acid. This turned out to be the key step in returning accurate results. We did this using the not-so-precise method of measuring 1mL of the original solution of 1 part oleic acid and 10 parts pentane, and moving it into another test tube of 10mL pentane. This was repeated two more times.
The next step was preparing a surface to allow the oleic acid to spread out so that it was only one-molecule tall. This involved using a large convex watch-glass, pouring water onto the top until there was enough surface tension to create a near-perfectly-flat surface. 0.05mL of the dilute solution was then dropped onto this surface. Because pentane is so volatile, it evaporated quickly leaving only the oleic acid. We were able to see this because of the light dusting of Lycopodium powder which was forced out of the way of the oleic acid.
Using the rough technique of sketching the outline of the oleic acid onto a plate of glass we were able to determine the area. By setting a simple proportion to find the weight to area ratio for printer paper, we solved for the approximate area of the oleic acid.
Using a formula provided, we calculated the true volume of the oleic acid in cubic centimeters or milliliters. The next step was to finally solve for the number of molecules. We know that by multiplying molar mass times the inverse density times the inverse volume, we can find molecules (g/mol x mL/g x mL/molecule). Our first trial proved to be the most accurate: we determined there were 4.9 x 1023 molecules of oleic acid, an error of approximately -1.1 x 1023.
There are two major sources of error in this experiment. The first is in the creation of the diluted solution. Measuring 1mL accurately with a...