Average Atomic Mass.

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Presentation transcript:

Average Atomic Mass

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.

Isotopes Atoms of the same element that have different # of Neutrons are said to be ISOTOPES of that element All isotopes of an element are chemically the same, i.e., they react exactly the same

Isotopes All elements found in nature are MIXTURES of isotopes Isotopes of an element are designated by mass number in either of two ways 1: 23592U 2: U-235 or Uranium-235 Using the above how would we designate the isotope of carbon that has 8 neutrons? 146C or C-14 (Carbon-14)

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.

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?

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!

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, and … 6.7% of the test grades are 60. Weighted Average = [.933 x 90] + [.067 x 60] = 83.97 + 4.02 = 88%

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

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

Avg. Atomic Mass of Chlorine Chlorine has two isotopes Percent Abundance Mass Isotope 1 75.770% 34.969 amu Isotope 2 24.230% 36.966 amu

Avg. Atomic Mass of Chlorine = Convert percents to decimals. (.75770)(34.969 amu) + (.24230)(36.966 amu) = 26.496 amu + 8.957 amu = 35.453 amu Multiply by appropriate mass. Sum

Avg. Atomic Mass of Chlorine To estimate the answer: 75% Cl is 34.969 and 25% Cl is 36.966. The final answer has to be between 34.969 and 36.966, but closer to 34.969. 35.453 amu

Avg. Atomic Mass of Si 92.21% of Si has a mass of 27.97693

Avg. Atomic Mass of Si .9221 X 27.97693  25.7975 .0470 X 28.97649  1.3619 + .0309 X 29.97376  0.9262 28.0856

Check your work! Answer is 28.0856. Answer has to be between high & low. Answer has to be closest to 27.97693. Answer is 28.0856.

Avg. Atomic Mass of Pb 1.5% Pb-204.  23.6% Pb-206.  22.6% Pb-207. 

Avg. Atomic Mass of Pb 1.5% Pb-204.  .015 X 204 : 3.06 207.24