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Atomic Masses

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  1. Atomic Masses • Using proportions in which elements combine, scientists determined relative atomic masses

  2. Atomic Masses • Picked the element carbon to be standard • 12C is assigned a mass of exactly 12 atomic mass units

  3. Atomic Masses • Mass spectrometer- used to compare masses of atoms by forming positive ions and then shooting them through a magnetic field

  4. Atomic Masses • The amount of path deflection depends on an atom’s mass • Example: when 12C and 13C are analyzed, their mass ratio is 1.0836129

  5. Atomic Masses • Mass of 13C = (1.0836129) x (12 amu) = 13. 003355amu • So, if 12C is our standard why is its atomic mass 12.01 and not 12.00?

  6. Atomic Masses • The law of averages!!! • The atomic mass on the periodic table is an average of all the isotopes of that element based on the percentage in an avg sample

  7. Ave Atomic Mass Is? • An element consists of 90.51% with a mass of 19.992 amu, .27% of an isotope with a mass of 20.994 amu, and 9.22% of an isotope with a mass of 21.990 amu.

  8. Atomic Mass Sol’n • (.9051)(19.992)= 18.09 • (.0027)(20.994)= .057 • (.0922)(21.990)= 2.03 • Sum of above 20.18

  9. M & M Problem • What would be ways for me to calculate for each of you 57 M & M’s? • Counting • Weighing

  10. M & M Problem • If we found the average mass of an M & M, we could then weight out very close to 57 M & M’s quickly. • No M & M’s may have exact mass- only need average

  11. Every Chemist’s Favorite Unit • Samples of matter usually contain lots of atoms • Our “standard” is based on how many atoms are in 12 grams of pure 12C

  12. Every Chemist’s Favorite Unit • This unit has a very special name that you will all learn to love dearly… It is called…

  13. Every Chemist’s Favorite Unit

  14. Every Chemist’s Favorite Unit • In 12 grams of 12C there are 6.02 x 1023 atoms • This is our definition of the MOLE!!

  15. Some Mole Facts... • 1 mole of seconds is 4 million times as long as the earth has existed!!! • 1 mole of marbles would cover the entire earth 50 miles deep!!!

  16. Some Mole Analogies • A mole is much like using the word “dozen” • We know dozen means 12 • We know mole means 6.02 x 1023

  17. Some Mole Problems! • You have a gold ring that is 10.94g. • How many moles is this? • How many atoms is this?

  18. Some Mole Problems! • A sample contains 5.00 x 1020 particles of iron. • What fraction of a mole is this? • What is the mass of this?

  19. Molar Mass Is... • Mass in grams of one mole of a COMPOUND! • Term also used is molecular weight

  20. Molar Mass • To find the molar mass of a compound, sum the masses of the component elements

  21. Molar Mass Problems • Calculate the molar mass of acetic acid. • What is the formula?

  22. Molar Mass Problems • You are given 89.72g of acetic acid. • How many molecules of acetic acid are there? • How many C atoms are there?