Avogadro's number

On 19/04/2011 00:07, Tom wrote: Trying to remember my Chemistry A level from 25 years ago.... I believe that an Atomic Mass Unit (AMU), is defined as 1/12 of the mass of a Carbon 12 neucleus. The reason for choosing C-12 is that it has an equal number of protons and neutrons, so the slight difference in mass is averaged out. In the past Hydrogen was used but since (the most common isotope) of Hydrogen only contains 1 proton, this lead to descrepancies for any nucleus with neutrons in. A Mole is really just an abitary amount of a substance that contains the same number of particals as a mole of any other substance, that is Avigadro's number. This is usefull when working with chemical formulas such as : 2 H2 + O2 ----> 2 H2O You know that s molecules of Hydrogen react with 1 molecule of Oxygen to give 2 molecules of Water. Or to put it another way 2 moles of H2 with 1 mole of O2 produce 2 moles of water. More practically 4 grams of Hydrogen + 32 gamms of Oxygen produce 36 gramms of Water. I think I rememberd it correctly...... Cheers. Phill. -- Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric ! "You can twist perceptions, but reality won't budge" -- Rush.