Let's look at an example of how we would use this information.

Here in this problem we're given grams of aluminum chloride and

we're looking to find how many chlorine atoms we have.

So, we know we're going to have to start with a 25.1 grams of

aluminum chloride because it's the only number we're given in the problem.

Now, I can't go from grams directly to atoms.

We have to go through mols.

And this will be a common theme both looking at compounds, but also when we

start looking at reactions, that the first thing we do is we convert to mols.

So in order to get from grams of aluminum chloride to moles of aluminum chloride,

I'm first going to have to know the molar mass.

So I see I have one aluminum, which has a mass of 27.00,

and I have three chlorines, each with a mass of 35.45.

Now I can find the molar mass by simply adding these values together.

So 27 plus 3 times 35.45 equals 133.35,

and remember this is a molar mass, so

it has units of grams per mol of aluminum chloride.

And notice that where I started my problem, I have grams in the numerator.

So I'm going to have to have grams in the denominator of the next step.

So I have 133.35 grams per mol.