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c161l6_09_15

# c161l6_09_15 - Chapter 3 Stoichiometry Chemical...

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Chapter 3 Chapter 3 Stoichiometry: Chemical Calculations

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Isotopes Isotopes W Different “varieties” of the same element W Which element is determined by Z, the number of protons W Which isotope is which is determined by A W All isotopes of the same element have very similar chemical properties
Isotopic mass and Natural Abundance Isotopic mass and Natural Abundance W Atomic mass is the average of the isotopic masses W A = Σ f i a i f i is the natural abundance of isotope i a i is the mass of isotope i W Need another equation 1 = Σ f i

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Example Example W Boron has 2 isotopes: 10 B with mass 10.0129u 11 B with mass 11.0093u W What is the natural abundance of 10 B? W Solution 10.81 = f 10 (10.0129) + f 11 (11.0093) 1 = f 10 + f 11 10.81 = f (10.0129) + (1- f )(11.0093) f = 0.1991 or 19.91%
The mole The mole W Individual atoms are in general too tiny to work with W Need a larger unit We buy eggs by the dozen Paper by the ream (about 500 sheets) Factories use the “hundred-count” Chemistry uses the mole W One mole is the number of things that is the same as the number of atoms in exactly 12g of 12 C

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c161l6_09_15 - Chapter 3 Stoichiometry Chemical...

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