What’s a mole?

A non-chemist’s view

A mole tells us “how many?” You know what your neighbor means

when they ask to borrow a dozen eggs--they want twelve eggs

A mole tells us “how many?” You know (or could figure out!) what

Abraham Lincoln meant when he said “four score and seven years ago…” during the Gettysburg Address---a score is 20 years

A mole tells us “how many?” You may have even heard of a gross--

which is a dozen dozen, or 144 of something

A mole tells us “how many?”What if you need 1 mole of the sugar,

glucose?

A mole of a compound is 6.02 X 1023 molecules

Let’s use some of these counting numbers…

Suppose that we had a huge, huge crate of eggs, and we wanted to know how many eggs we had. How could we do this without counting?

FRAGILE--EGGSNet wt = 50 lb

The eggs problem

We can solve this problem, since we know the weight of the contents of the crate…

EggsIf I knew how much a dozeneggs weighs, I could solve this!

The eggs problem

Imagine that we know that a dozen eggs weighs 1 lb.

Eggs If a dozen eggs weighs 1 lb, I can set up a proportion…Net wt = 1 lb

Setting up a proportion

Here we set up a proportion to determine how many dozen eggs are in the crate, based upon weight.

FRAGILE--EGGSNet wt = 50 lb

Eggs

1 dozen eggs = z 1 lb 50 lb

The eggs solution

Here we solve our proportion to determine how many dozen eggs are in the crate.

FRAGILE--EGGSNet wt = 50 lb

Eggs

(1 dozen eggs)(50 lb) = z 1 lb50 dozen eggs = z!!!

But, why do we care what a mole is?

So a mole of something is like a dozen:– A dozen eggs is 12 eggs– A mole of glucose is 6.02 X 1023 molecules of

glucose. Why do we care about moles? Because

concentrations of solutions are often expressed in terms of molarity

Molarity = moles of a compound liter of solution

Here’s why we care…

Suppose that I need to make a 0.1 M (molar) solution of glucose…

Great! I know that a 0.1 M solution requires 0.1 moles of glucose for each liter of solution!

Hmmm…Molarity = moles of a compound

liter of solution

Here’s why we care…

So… I need a whole lot of molecules of glucose per liter. What now ???How am I going to measure that out?!?

Well, I know that 1 mole of glucose is 6.02 X 1023 molecules, so 0.1 moles must be 0.1 times that number…..

How do we get from moles to something we can measure, like

weight?

The key is that every compound or element has its own unique weight

So a mole of every type of molecule has the same number of molecules (6.02 X1023), but a different weight…

Let’s return to our eggs analogy

Eggs are sort of like elements in the periodic table…Lots of similarities between eggs…

Eggs… Elements…

ProtonsNeutronsElectrons

ProtonsNeutronsElectrons

ProtonsNeutronsElectrons

• Lots of similarities between elements!!

But there are differences too!

Eggs… Elements…

4 Protons 5 Neutrons 4 Electrons

2 Protons2 Neutrons2 Electrons

3 Protons4 Neutrons3 Electrons

So, the weight of a dozen eggs depends upon the type of egg, and the weight of a mole of an element depends upon the type of element!

Eggs come in different sizes…medium, large, extra large…Elements have different numbers of protons, neutrons, electrons

Back to our egg problem…

Remember, we want to determine how many eggs are in the crate, without counting them all!!

FRAGILE--EGGSNet wt = 50 lb

Eggs

The eggs problem

We can solve this problem, since we know the weight of the contents of the crate…

Eggs If I knew how much a dozeneggs weighs, I could solve this!

The eggs problem

Of course, we can only know how much a dozen eggs weighs, if we know what type of egg…

Med Eggs

Net wt = 0.75 lb

Lrg Eggs XL Eggs

Net wt = 1 lb Net wt = 1.25 lb

The eggs problem

Imagine that we know that our crate is filled with Large eggs. A dozen Large eggs weighs 1 lb.

Lrg Eggs If a dozen Large eggs weighs1 lb, I can set up a proportion…

Net wt = 1 lb

The eggs solution

Here we solve our proportion to determine how many dozen Large eggs are in the crate.

FRAGILE--LARGE EGGSNet wt = 50 lb

Lrg Eggs

(1 dozen Large eggs)(50 lb) = z 1 lb50 dozen Large eggs = z!!!

The key to solving this problem is: Knowing how much a dozen of the correct type of egg weighs!

Making solutions using molarity

Fortunately, we can, for any compound, figure out how much 1 mole of that compound weighs: 1 mole of any element or compound is equal to its molecular weight expressed in grams (we can look this molecular weight up in books, the internet, on chemical bottles…)

Back to our glucose solution…

Remember, we need 0.1 moles of glucose for each liter of solution

The molecular weight of glucose is 180.So 1 mole of glucose must weigh 180 grams!!

Hmmm….Let’s look at the bottle:

GlucoseMW 180

Making our glucose solution…

Remember, we need 0.1 moles of glucose for each liter of solution

So I must need: 0.1 moles X 180 grams

1 moleor 18 grams of glucose for every liter!!