MadSci Network: Chemistry |
Avogadro's number is officially defined as the number of particles in 12.0 grams of Carbon-12 (carbon that has 6 protons and 6 neutrons in its nucleus). The accepted value today is approximately 6.0221367*10^23, and with the equipment available to a modern chemist there are several ways to arrive at this number. A chemist could use a mass spectrometer, a precise instrument which can determine an element or compound's mass. We won't go into details, but such a device is sensitive to the number of atoms present, and it could be used for "atom counting." Of course, a chemist wouldn't count out an entire mole of atoms, but could count a small amount and then using proportions could find out how many atoms are in a mole. Also, one could use the theory behind Brownian Motion to calculate Avogadro's number. Albert Einstein did this successfully in 1905. Brownian Motion occurs when small solvent molecules collide with larger solute molecules. (You can observe this by placing a drop of milk under a microscope). Finally, one could determine Avogadro's number using x-rays. By examining how x-rays bounce off of a crystal, a chemist can tell how atoms in the crystal are spaced. By using the density of the crystal and the fact that density = mass/volume, one can calculate Avogadro's number. The actual formula used is the following: Na = (nM)/(pV) In this case Na = Avogadro's number, n is the number of atoms in a crystal cell, M is the molar mass of the element, p is the element's density, and V is the volume of a cell. A cell is the smallest repeating structure of a crystal. Also, you can check the following site for more information and additional methods for calculating Avagadro's number: http://www.madsci.org/posts/archives/jan99/916465856.Ch.r.html Any college level text book will also contain information on at least one method of calculating Avogadro's number.
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