November 9th - Avogadro's Number

Today was the day we started our second unit: The Mole. We started the class by getting our tests and labs back. We then dove right into the notes for our fresh topic.

As we are well aware from previous studies, atoms and molecules are extremely small. Macroscopic (visible to the naked eye) objects contain way too many atoms/molecules for us to obtain a practical number for the weight or count of the sample. Thankfully, an Italian chemist by the name Amedeo Avogadro thought of a way to represent a large number of atoms with one convenient unit. Behold Avogrado:

Avogadro proposed that the number of atoms in 12.00000g of carbon-12 is equal to a constant. This constant would be known as a mole. Therefore, in 12.00000g of carbon-12, there is 1 mol. This constant can also be applied to other samples to gain a measure of its relative number of atoms!

This value, which is equal to 6.02 x 10²³ atoms, is now used as the basis for all quantitative chemistry!

 

To demonstrate how big this number really is, we were introduced to the pea analogy. 

According to the analogy, if we had one million peas, we could fill an average bedroom. That's a large number of peas, but it's practically nothing compared to a mole of peas. In fact, one million is only 0.000000000000000166113% of a mole. 

Let's jump up a few exponents. If we had one sextillion of peas, we could cover the entire surface of the earth with peas measuring 1 metre deep. Still, that's only 0.0016% of a mole.

Now, let's say that we cover 602 planets, all the same size as Earth, with peas measuring 1 metre deep. That, my friend, is one mole. And that is the same number of atoms that exist in 12 grams of carbon-12.  

To convert between moles and atoms, we can use simply use dimensional analysis. Here's an example:

If we have a sample of iron containing 5.05 x 10²⁴ atoms, how many moles of iron do we have?

We can also use dimensional analysis to convert between moles and ionic or covalent compounds. Here's an example:

If we have a sample containing 7.0 moles of bromine (Br₂), how many molecules do we have?

 

If we are converting to find how many atoms make up x number of moles, we would answer using the element symbol (ex.          moles of Fe).

If we are converting to find how many molecules (of a covalent compound) make up x number of moles, we would answer using the chemical formula (ex.          moles of HOH).

If we are converting to find how many formula units (of an ionic compound) make up x number of moles, we would also answer using the chemical formula (ex.          moles of NaCl).

Here's a nice video explaining the mole in detail:
 

 

Posted by Michael.