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Average Atomic Mass

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How much does an atom weigh? Protons & Neutrons: 1.67 X 10 -24 gram Electrons: 9.10 X 10 -28 gram To avoid working with these really small numbers, scientists have developed the atomic mass scale.

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What is an a.m.u.? atomic mass unit amu 1/12 the mass of the C-12 atom. C-12 is used as the reference for atomic masses. This is the STANDARD.

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Atomic Weight or Mass Atomic mass is relative. C-12 is the standard. 1 atom of C-12 has a mass of exactly 12.000… atomic mass units. So all the atoms are compared to C-12.

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Average Atomic Mass The atomic masses reported in the periodic table represent the weighted average of the masses of the naturally occurring isotopes of that element.

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Weighted Avg. of Test Grades Suppose you have 1 test of 60% and 14 tests of 90%. What if I said your avg. was 75? 60 + 90 = 150. Divide by 2 = 75. Would you be happy?

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NO! The long way to do this problem: (90 + 90 + 90 + 90 + 90 + 90 + 90 + 90 + 90 + 90 + 90 + 90 + 90 + 90 + 60) 15 Avg. = 88. You like this a lot better!

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The smart way to do this problem! 14 out of 15 is 93.3% 1 out of 15 is 6.7% So 93.3% of the test grades are 90. 6.7% of the test grades are 60. .933 X 90 +.067 X 60 = 83.97 + 4.02 Avg. = 88

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This is an even smarter way to calculate the average if you have lots of items to average.

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Method of Weighted Averages 1. 1. Convert % to decimal format. (Divide by 100%.) 2. 2. Multiply each isotope’s abundance factor by its atomic mass. 3. 3. Sum.

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Avg. Atomic Mass of Chlorine Percent Abundance Mass Isotope 1 75.0%35 amu Isotope 2 25.0%37 amu Chlorine has two isotopes.

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Avg. Atomic Mass of Chlorine = (.75) (35 amu) + (.25) (37 amu) (.25) (37 amu) = 26.25 amu + 9.25 amu = 35.5 amu Convert percents to decimals. Multiply by appropriate mass. Sum

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Avg. Atomic Mass of Chlorine To estimate the answer: 75% Cl is 35 and 25% Cl is 37. The final answer has to be between 35 and 37, but closer to 35. 35.5 amu

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Avg. Atomic Mass of Si 92.21% of Si has a mass of 28 4.70% of Si has a mass of 29 3.09% of Si has a mass of 30

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Avg. Atomic Mass of Si.9221 X 28 25.8188.9221 X 28 25.8188.0470 X 29 1.363.0470 X 29 1.363 +.0309 X 30 0.927 +.0309 X 30 0.927 28.1088 28.1088

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Check your work! Answer has to be between high & low. Answer has to be closest to 28. Answer is 28.11.

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Avg. Atomic Mass of Pb 1.5% Pb-204. .015 X 204 : 3.06 23.6% Pb-206. .236 X 206 : 48.62 22.6% Pb-207. .226 X 207 : 46.78 52.3% Pb-208. .523 X 208 : 108.78 207.24

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