The Mole   

Teacher's notes for the Mole mini-lesson


Who, What, When, and Why?

Who:  A German chemist named Wilhelm Ostwald created this term. 

What:  A term used to describe the amount of a substance.  Actually equal to 6.022 x 1023 particles, atoms, or molecules of a substance.

When:  This term was first used around 1896.

Why:  The purpose of this term is so scientists can know exactly how many atoms or molecules are reacting together and what product they form.

Alright, so this still doesn't make any sense, lets try a comparison...

A scientist uses the word "mole", like a baker uses the word "dozen".  It stands for a certain amount of a substance.  

A mole= 6.022 x 1023 particles, while a dozen = 12 particles.

So, if you have 2 dozen doughnuts, you have 2 x 12, or 24 doughnuts.

If you have 2 moles of water, you have 2 x 6.022 x 1023 water molecules..

Now that we understand that the mole is just a way to describe how much of something we have, what can we do with this information?

Well, lets say you are working in a lab and you need 20.0 grams of Iron (III) hydroxide.  The only compounds you have available are Iron (III) chloride and Sodium hydroxide.  You know, however, that these two compounds react in the following fashion:

FeCl3(aq)  +  3NaOH(aq)  -->  Fe(OH)3(s)  +  3NaCl(aq)

Lets Make sure we understand what this chemical equation is telling us.  The numbers before each compound tell us the mole ratio of that compound in the reaction.  If there is no number, we assume the number is one.  So, this reaction is telling us that we need 1 mole of  iron (III) chloride to react with 3 moles of sodium hydroxide to produce 1 mole of iron (III) hydroxide.

Ok, so we understand what the reaction is saying, but how do we use this information to get the 20.0 grams of iron (III) hydroxide we need.  Well, we definitely can't separate and count out the molecules of each compound we need, se we need another way....USE THE INFORMATION ON THE PERIODIC TABLE TO FIND THE MOLES OF EACH COMPOUND.  This is done by calculating the molar mass.  We need to use the molar mass (units= grams/moles) for each element to get the molar mass (units= grams/moles) for each compound:                

Iron (III) chloride= 162.2 grams/mole        

Sodium hydroxide= 39.99 grams/mole                         

Iron (III) hydroxide= 106.9 grams/mole                        

We don't really need the molar mass of sodium hydroxide because we don't care how much of it we produce, we are only interested in the two reactants we need to mix together in our lab to get the 20.0 grams of iron (III) hydroxide.

Once we have the molar mass for each compound in the reaction, we can find the moles for each compound.  

Alright, now after all this talk, it's time for the calculation...  We want 20.0 grams of iron (III) hydroxide which has a molar mass of 106.9 grams/mole. So we need to calculate how many moles of Fe(OH)3(s) we need in our lab.  Once we find out how many moles of Fe(OH)3(s)  we need we can use the mole ratios in the reaction to figure out how many moles of each reactant we need to use to get our 20.0 grams of Fe(OH)3(s)

20.0 grams Fe(OH)3(s)  x  1mole/ 106.9 grams  =   0.187 moles Fe(OH)3(s)

Now that we have how many moles of iron (III) hydroxide we need, let's figure out how many grams of iron (III) chloride we need.  Well, we want 0.187 moles of iron (III) hydroxide, for every 1 mole of iron (III) hydroxide produced, we use 1 mole of iron  (III) chloride which means we need the same amount of moles 0.187  moles.  Now lets convert to grams.  If we need 0.187 moles and the molar mass of iron (III) chloride is 162.2 grams/mole, then we need 30.3 grams from our bottle.

0.187 moles Fe(OH)3 x 1mole FeCl3/ 1mole Fe(OH)3 x 162.2grams FeCl3/ 1mol FeCl3

30.3 grams FeCl3

Now lets figure this out for sodium hydroxide.   We need to produce 0.187 moles of iron (III) hydroxide and for every 1 mole produced we need 3 moles of sodium hydroxide, so we need 3 times as many moles.  Now lets convert to grams.  We need 0.561 moles and the molar mass of sodium hydroxide is 39.99 grams/mole, so we need 22.4 grams of sodium hydroxide from our reagent bottle.

0.187 moles Fe(OH)3 x 3mole NaOH/1mole Fe(OH)3 x39.99 grams NaOH = 22.4 grams NaOH

So, our conclusion is that to get 20 grams of Fe(OH)3, we need to react 30.3 grams of FeCL3 with 22.4 grams of NaOH.  

There are two important pieces of information to take away from this lesson besides just knowing what a mole is.  The first is always write down your units and make sure they cancel out to give you the units you are looking for.  Second,  a reaction tells us mole ratios, so always make sure you convert to moles using the mole ratios before changing from one compound to another.

Well done, So now we know what the mole is and why we need to know about it.  Try some of these other problems to make sure you understand the calculations.  And remember, always write down your units.  If your units come out right, most likely your answer is correct.

If you had difficulty understanding this problem, go to the mini lesson on stoichiometry.  The stoichiometry mini-lesson goes into more detail on using the mole to figure out calculations using a balanced chemical reaction.