A mole is simply another word for a certain number of things.

A mole is simply another word for a certain number of things.

Just like: dozen is another word for 12 things pair is another word for 2 things ream is another word for 500 things gross is another word for 144 things

Well, mole is another word for 602 sextillion things 602 sextillion = 602,000,000,000,000,000,000,000 In scientific notation 602 sextillion is written as 6.02 x 1023 (Some textbooks write a mole with 4 significant figures as 6.022 x 1023 )

That's a lot of things!! So, why should you care about the mole?

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ATOMIC MASS UNITS and the MASS of a Single Atom

About 150 years ago (more or less) chemists determined that the mass of a single atom of any element could be expressed in a unit they called the ATOMIC MASS UNIT. The atomic mass of each element is located with the elements on the periodic table and is the "weight" or mass of a single atom of that element expressed in atomic mass units. So, looking at the periodic table, we find that

A single carbon atom weighs (or has a mass of ) 12.0 amu A single nitrogen atom weighs (or has a mass of ) 14.0 amu A single calcium atom weighs (or has a mass of ) 40.1 amu

The problem is, we still don't have an instrument that measures out amu's (atomic mass units). So, how does this help us?

These chemists also determined that one atomic mass unit weighs (has a mass of) 1.66 x 10-24 grams. So, for carbon, 12.0 amu x 1.66 x 10-24 = 1.99 x 10-23 g.

Now the problem is, we do have instruments that can measure out grams, but none of them can measure out something as small is 1.99 x 10-23grams so we still can't measure out 1 carbon atom. (Here's where the mole comes in!!)

Chemists are very fond of the mole because it helps them measure out the right number of atoms or molecules for

their chemical reactions.

Remember that chemical reactions happen atom per atom or molecule per molecule. Balancing equations tells us how many atoms or molecules we need of each reactant to complete the reaction and end up with the proper number of products.

For example: It takes 2 molecules of hydrogen gas and 1 molecule of oxygen gas to produce 2 molecules of water. The equation is: 2H2 + O2 2H2O

But, we can't even see 2 molecules of H2, much less count them out. We can't even see 2 billion molecules of hydrogen gas or oxygen gas. We can see a drop of water or perhaps a cc or mL of water. But how many atoms or molecules are in a drop or in a mL???? Again, THAT is the problem!

The mole is how we solve that perplexing problem! Here's how it works

If you do the math, you learn that a mole of any element is equivalent to it's atomic mass in grams.

Using Iron as our example, we know from the periodic table that the mass of one iron atom = 55.8 amu (atomic mass units).

We also know that 1 amu has a mass of 1.66 x 10-24 g.

Iron

26

Fe 55.8

Atomic mass for Iron

So, the mass in grams of one iron atom =

55.8 amu x 1.66 x 10-24 g = 9.26 x 10-23 g Fe (iron)

1 amu

Recalling that a mole = 6.022 x 1023 things, we can do the math (below) and learn that a mole of iron atoms has a mass of 55.8 g

The math.... ( 9.26 x 10-23 g (weight of 1 Fe atom) x 6.022 x 1023 atoms = 55.8 g.

You should recognize that number!! Look back at your periodic table at iron. As it turns out, a single atom of iron has a mass of 55.8 amu AND, quite conveniently, a mole of iron atoms weighs 55.8 grams!

The really great part is that YOU don't have to do the math!!

Just remember that 1 mole (6.022 x 1023 atoms) of ANY element or molecule is equal to the atomic mass expressed in grams.

Check it out on your periodic table! 1 atom of boron (B) weighs 10.8 amu 1 atom of radium (Ra) weighs 226.0 amu 1 atom of copper (Cu) weighs 63.5 amu 1 atom of iodine (I) weighs 126.9 amu

1 mole of boron (B) weighs 10.8 g 1 mole of boron (B) weighs 226.0 g 1 mole of copper (Cu) weighs 63.5 g 1 mole of iodine (I) weighs 126.9 g

Now we have a way to measure out the amount of any element or molecule we need for our reactions!

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