Italian lawyer turned scientistFamous for proposing that equal volumes of gases held at

same temperature and pressure must have the same number of gas particles (Avogadro’s Hypothesis-1811)

Several scientist attempted to measure how many gas molecules are in a specific volume of gas

Robert Millikan determined the charge of the electronWhen scientists divided the charge of a set amount of

electrons by the charge of a single electron they calculated the number 6.022 x 10 23 aka Avogadro’s Number (NA)

Chemists compared the common Carbon-12 isotope to the smallest element Hydrogen-1.

Carbon-12 was 12 times as “heavy” as Hydrogen-1, so chemists made it easy on themselves and said that Hydrogen = 1 atomic mass unit (amu) and Carbon = 12 amu

Some other comparisons to Hydrogen

Calcium = 40 amu (40x heavier than H-1)

Sulfur =32 amu (32 x heavier than H-1)

Once Chemists had relative masses of all the elements they could determine how much molecules “weighed” by adding all the amu values for all the elements in a molecule-this is called formula mass.

H2O Formula Mass2 Hydrogen atoms x1 amu

for each H atom = 2 amu1 Oxygen atom = 16 amu2 amu (from H) + 16 amu

(from O) = 18 amu1 Water molecule has a

mass of 18 amu

Example of Formula Mass

Ca = 40 amuS = 32 amuO=4 oxygen atoms x 16 amu each = 64 amu40 amu + 32 amu + 64 amu = 136 amu1 molecule of Calcium sulfate “weighs” 136 amu

Chemists came up with a quantity for comparing the amounts of substances and called it “The Mole”

1 Mole of any substance is called Avogadro’s Number 6.022 x 10 23.

Insanely huge Number!! 1 mole of marshmallows

would be enough marshmallows to make a 12 mile thick layer of marshmallows covering the entire face of the Earth!!

A pair = 2 thingsA dozen = 12 thingsA gross = 144 thingsA mole = 6.022 x 10 23

things

The amu is far too small for chemists to measure in lab.Since all element masses were relatively compared to

each other, chemist use the mass unit gram to measure solids in the lab.

This is really convenient! Since Hydrogen is 1 amu and Carbon is 12 times as “heavy” at 12 amu, chemists changed the value of amu to gram so that they could measure in lab.

One mole of H = 1 gram (This is the molar mass of H)One mole of C =12 grams (Carbon molar mass)

Ca = 40 gramsS = 32 gramsO=4 oxygen atoms x 16 grams each = 64 grams40 grams + 32 grams + 64 grams = 136 grams

Does this look familiar??1 molecule of Calcium sulfate “weighs” 136 amu1 mole of Calcium sulfate “weighs” 136 grams

If you had one dozen dump trucks, pencils, and babies (yeah babies why not?) you would have 12 of each type of thing, but what would be different about them?

Their Masses would be different.If you had one mole dump trucks, pencils, and pickles,

you would have 6.022x 10 23 of each but they would not “weigh” the same!!

Make Sense?!Happy Mole Day to Youhttp://www.youtube.com/watch?v=ReMe348Im2w