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1 Average Atomic Mass Chemistry Notes

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2 Relative Atomic Mass Masses of atoms expressed in grams are very small, for example: One atom of Oxygen-16 has a mass of 2.66x10 -23 g = 0.0000000000000000000000266g The standard used to denote units of atomic mass is the carbon-12 nuclide. It has been arbitrarily assigned a mass of exactly 12 atomic mass units

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3 Relative Atomic Mass One atomic mass unit, or amu, is exactly 1/12 of a carbon-12 atom. Atomic Mass is determined by comparing it with the mass of a carbon-12 atom. Therefore, atomic mass is a measure of an atom’s mass relative to carbon- 12.

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4 Average Atomic Mass Most elements occur naturally as a mixture of isotopes. The percentage at which each isotope occurs in nature is taken into account when calculating the element’s average atomic mass. Average Atomic Mass- the weighted average of the atomic masses of the naturally occurring isotopes of that element.

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Compare average atomic mass to mass number Mass number is the sum of the protons and neutrons of ONE isotope of an element (whole number) Average atomic mass is the weighted average of ALL of the isotopes of an element (usually not a whole number b/c its an average) 5

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6 How Do You Calculate a Weighted Average? You have a box of 100 marbles. 25 of the marbles have a mass of 2.0g each. 75 of the marbles have a mass of 3.0g each.

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7 Weighted Average Calculation CCalculate the total mass of the mixture & divide by 100. 25 x 2.0g = 50g 75 x 3.0g = 225g TTotal mass = 275g 2275g ÷100 = 2.75g (average mass of marbles)

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8 Calculate the Average Atomic Mass of an Element (amu) IsotopeMass Number % Natural Abundance Atomic Mass (amu) Copper-636369.17% (69.17 copper atoms out of 100 are this isotope) 62.93 Copper-656530.83% (30.83 out of 100 are this isotope) 64.93

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9 Calculate the Average Atomic Mass of an Element (amu) CCopper-63 69.17 x62.93 amu = 4353 amu CCopper-65 30.83 x 64.93 amu = 2002 amu TTotal Mass = 6355 amu 66355 amu ÷ 100 = 63.55 amu WWhat is Copper’s Atomic Mass on the periodic table?

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10 Calculate the Average Atomic Mass of Oxygen IsotopeMass Number % Natural Abundance Atomic Mass (amu) Oxygen-16 1699.7615.995 Oxygen-17 170.03816.999 Oxygen-18 180.20017.999

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11 Answer 16.00 amu

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Formula to calculate avg atomic mass Multiply the mass of each isotope by its percent abundance and then add them all together; then divide the total by 100 (Mass of isotope 1 x % abundance) + (mass of isotope 2 x % abundance)+ 100 12

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13 Which isotope is most abundant? The average atomic mass of an element is always closest in value to the most abundant isotope. For example, Carbon’s average atomic mass is 12.011, so the most abundant isotope of carbon is carbon-12. The average atomic mass of sulfur is 32.06. The isotopes of sulfur are sulfur-32, sulfur-33, sulfur -34, and sulfur-36. What is the most abundant isotope of sulfur?

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14 Sulfur-32

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15 Assignment Pg 117 practice problems #23-24 Pg 124 problem #81

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