Stoichiometry

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If you have 3 jelly beans, then you can count it easily. If you have 100 jelly beans, counting will take some time. If you have 10,000,000 jelly beans, counting them would take nearly forever.

Stoichiometry

Stoichiometry is used in chemistry for counting molecules that are too numerous to count. To understand this, first we need to know some concepts.

Avogadro's Principle

Avogadro's Principle says that, for any gas at STP, the same molecules of gas will always take up the same volume, no matter how many molecules there are or what the gas is. 

The Mole and Avogadro's Number

The mole is defined as 6.022 x 10^23 molecules. Why this number? This number is the number of molecules in 12 grams of Carbon-12. 

Question: How many molecules are in 3 moles of a substance? What is the mass of 3 moles of Carbon-12?

Answer (Highlight to see): There are  18.066 x 10^23 molecules. The mass is 36 grams.

The significance of this is that 1 mole of an atom with atomic mass x amu will weigh x grams.

For example, 1 mole of Carbon-12, which has an atomic mass of 12 amu, will weigh 12 grams.

Question: How much grams does 3 moles of Carbon-12 weigh? Carbon-12 weighs exactly 12 amu.