Reminder: 1 mol of anything e.g. Oxygen, carbon, dollars, toys, etc. is 6.02 x 1023

Let’s look at this pile of carbon in the form of charcoal. I bet you there is a bunch of carbon moles in there. Let’s see how much.

Now, this clump represents one mole. Can you estimate how many do you see in the whole bunch? Well, it’s hard to visualize. Let’s do some math.

Since this is 1 mole, that means it weighs 12.01 g. Why? Because, according to the periodic table, 1 mole of Carbon weighs 12.01 g. Well, how many moles are present in this whole bunch? Let’s weigh it first.

0.00 g12.01 g

6.02 x 1023

0.00 g

So, we weigh the whole bunch and it turns out to be 15.50 g.

15.50 g

1 mol

? mol

So, there are two ways to solve this problem:

a) Use proportions

b) Use the n= m / mMm

a) Use proportions:

Since 1 mol of C = 12.01 g

? mol = 15.50 g

? mol = 1 mol of C x 15.50 g12.01 g

= 1.29 mol

b) Let’s use the formula n= m / mMm

Step 1: What do we have?

m: 15.50 g

mMm: 12.01 g/mol

N: ?

Step 2: You can use a triangle

m n: m . mMm

n x mMm n: 15.50 g 12.01 g/mol

n: 1.29 mol

Another example: let’s say there are 4 moles of water in this beaker. How many grams would that be?

Well, there are a couple of steps. We are trying to find the mass of 4 moles of water, we need the molar mass of water.

Step 1: Find molar mass

H2O: 2(1) + 16 = 18 g/mol

Step 2: Let’s find the mass, I’ll use the proportions method

1 mol of H2O = 18 g

4 mol of H2O = ? g

? g = 4 mol of H2O x 18 g/mol 1 mol of H2O

? g = 72 g

What about the number of molecules?

Well, for the number of molecules, let’s see what happens.

1 mol of H2O = 6.02 x 1023 molecules of H2O

4 mol of H2O = ? molecules

? molecules = 4 mol of H2O x (6.02 x 1023 molecules of H2O) 1 mol of H2O

= 2.408 x 1024 molecules of H2O

Okay, solve the following problems:

a) How many moles of Ca(OH)2 are present in a 200 g sample of Ca(OH)2?

b) How many molecules of MgF2 are present in a 100 g sample of MgF2?