1 / 18

Average Atomic Mass

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

malloy
Download Presentation

Average Atomic Mass

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Average Atomic Mass

  2. 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.

  3. 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

  4. 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)

  5. 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.

  6. 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?

  7. 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!

  8. 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%

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

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

  11. Avg. Atomic Mass of Chlorine Chlorine has two isotopes

  12. 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

  13. 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

  14. Avg. Atomic Mass of Si 92.21% of Si has a mass of 27.97693 4.70% of Si has a mass of 28.97649 3.09% of Si has a mass of 29.97376

  15. Avg. Atomic Mass of Si .9221 X27.97693  25.7975 .0470 X 28.97649  1.3619 + .0309 X 29.97376  0.9262 28.0856

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

  17. Avg. Atomic Mass of Pb 1.5% Pb-204.  23.6% Pb-206.  22.6% Pb-207.  52.3% Pb-208. 

  18. 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

More Related