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Chap 3

# Chap 3 - Atomic mass In order to do proper chemistry we...

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Atomic mass In order to do proper chemistry, we need to how many molecules we have. Since we can’t very well COUNT the molecules, it is convenient to weigh the sample, and divide by the mass of one molecule Molecules are to the order of 10^-26 kg. This is not convenient, so we created the atomic mass unit (amu) 1 amu = 1/12 mass of carbon-12

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Notes on amu’s One would reason that if carbon 12 had a mass of 12, then carbon-13 would have a mass of exactly 13. NOT TRUE! The issue is complicated because a) neturons weigh slightly more than protons b) electrons have a small weight, which adds up c) quantum physics shit I don’t even wanna TALK about Also, note that the atomic weight of carbon given is NOT exactly 12. This is because there are isotopes of carbon that need to be taken into consideration. The atomic weight shown on the periodic table is equal to the weighted average of the isotopes
The mole In a nut shell, the mole is the perfect conversion unit from amu’s to gram Ex. If one molecule weighs 5 amu’s, then one MOLE weighs 5 grams Remember that when we say “a mole of atoms” it’s just like saying “a dozen donuts”. It represents a number And that number is 6.02 * 10^23 Now, the atomic mass given on the periodic table also means “grams per mole”

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