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## Atomic Masses

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**Atomic Masses**• Using proportions in which elements combine, scientists determined relative atomic masses**Atomic Masses**• Picked the element carbon to be standard • 12C is assigned a mass of exactly 12 atomic mass units**Atomic Masses**• Mass spectrometer- used to compare masses of atoms by forming positive ions and then shooting them through a magnetic field**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**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?**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**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.**Atomic Mass Sol’n**• (.9051)(19.992)= 18.09 • (.0027)(20.994)= .057 • (.0922)(21.990)= 2.03 • Sum of above 20.18**M & M Problem**• What would be ways for me to calculate for each of you 57 M & M’s? • Counting • Weighing**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**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**Every Chemist’s Favorite Unit**• This unit has a very special name that you will all learn to love dearly… It is called…**Every Chemist’s Favorite Unit**• In 12 grams of 12C there are 6.02 x 1023 atoms • This is our definition of the MOLE!!**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!!!**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**Some Mole Problems!**• You have a gold ring that is 10.94g. • How many moles is this? • How many atoms is this?**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?**Molar Mass Is...**• Mass in grams of one mole of a COMPOUND! • Term also used is molecular weight**Molar Mass**• To find the molar mass of a compound, sum the masses of the component elements**Molar Mass Problems**• Calculate the molar mass of acetic acid. • What is the formula?**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?