The Mole and avagadro's number

Don't forget, you need avocado's number to do guacaMOLE calculations

The Mole is an SI unit to describe the amount of a substance.

1 mol = 6.022 x 10^23 objects

1 mol = 6..022 x 10^23 atoms (This is called Avagadro's Constant)

Molar Mass

You know how to calculate Average Atomic Mass, it's a weighted average measured in amu, and is on the periodic table for every element.

***Molar mass is measured in g/mole ***

This number on the periodic table also refers to the Molar Mass. Molar Mass is SUPER IMPORTANT. And it tells us the amount of grams that 1 mol of that substance weighs.

So, it's pretty easy if you just need the molar mass of a single element.

Sometimes, you need the molar mass of a compound -- in which you need to take into account how many of each atom you have.

For example, if I was calculating the molar mass of CaCl2...

Conversion Problems

Getting these molar masses are important, because they act as a conversion factor. Remember, molar mass tells us how many grams of that substance are in 1 mol. So now we can convert between grams and moles!

It's just normal train-track problems, but using the molar mass from the periodic table as the conversion factor between grams and moles.

We also know that there are 6.022 atoms in a mole -- so we can also calculate the amount of atoms in a substance!

So if I asked: How many atoms are in 3.91 mols of Xe? You don't need the molar mass -- that tells you how many grams are in a mole, and we don't have any grams in this problem. All you need is Avagadro's number!

Use this flow map!!!

If you're going between grams and moles, you're going to need molar mass, and if you're going between moles and atoms/molecules, you're going to need Avagadro's #.

Sometimes you start with grams, and have to go to atoms, which means you need to go through two steps.

Sometimes you start with atoms and have to go back to moles, and then back to grams (reference the flow map above)


1. How many moles are in 72.9 g of HCl?

2. How many moles of nitrogen are there in 4.3 × 10^23 molecules?


1. 1.999mol

2. 0.714 mol

3. 2.406 x 10^24

14. 478.93 g

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