Atomic Theory

The Mole

Things that you already know

• Science measures a lot of things.

• Examples of what we measure include time, mass, temperature, volume, speed, etc.

• We also have defined units to help us describe these measurements.

• Some of the terms that you know are seconds, grams, liters, meters, etc.

Something new:

• Science has a measurement that defines the “amount of a substance”.

• This is not the same as the mass or the volume.

• Instead, it defines a specific number of particles of that substance.

• By particles, we are talking about atoms of an element.

• We could also be talking about molecules of a compound.

So how many particles?

• A lot, actually.• The specific number is:

602,000,000,000,000,000,000,000 - and that is a rounded-off value.This number actually has a name – it’s called “Avogadro’s Number” –named after an Italian Chemist from the past.

Definition for us:

• The Avogadro Number of atoms of an element is called

1 mole of that element.

• Another way: that says that

602,000,000,000,000,000,000 atoms of any element is called 1 mole of that element.• It does not matter which element you have….

There is More !!!

• Suppose we counted out 1 mole of atoms of the element iron and placed them all on a balance at the same time.

• That would be 6.02 x 1023 atoms of iron.

What would all of those Atoms weigh???

• All together, the 6.02 x 1023 atoms of iron would have a mass of 55.847 grams.

Where did that number come from ??

If you look on the periodic chart, you see that this number is found in the box for iron.

It is the average atomic weight for this element.

This gives us a 2nd definition for the mole….

Definitions of the Mole

• We already have the definition that states 1 mole is equal to 6.02 x 1023 atoms.

• Now we get the definition that 1 mole is equal to the average atomic weight in grams.

• We get to use the appropriate definition we need to solve the given problem.

Consider this example:

What mass, in grams, is equivalent to 3.80 moles of sodium?

Solution:

3.80 moles Na ( 23 grams Na

1 mole Na)

= 87.4 grams NaThe 23 grams of Na came from the periodic chart – it is the atomic weight that is reported on the chart. Note too how the moles cancel. Always set up factors to cancel.

Consider this other example:

How many moles are equal to a mass of 200 grams of nitrogen?

Here is how this one works out.

200 grams N 1 mole N

14 grams N= 14.29

moles