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Moles

Content objective:

What are we learning and why are we learning this? Content, procedures, or skills.

Vocabulary objective

Tier II: High frequency words used across content areas. Key to understanding directions & relationships, and for making inferences.

Tier III: Low frequency, domain specific terms.

Building on what we already know

Make connections to prior knowledge. 

What is a “mole?”

It’s just a way of counting that makes it easier to solve practical chemistry problems.

We use moles to talk about extremely large numbers of very tiny objects (e.g. molecules, atoms or electrons.)

You already know that we use words to describe units, like

dozen = 12 of something

gross = 144 of something

ream = 500 sheets of paper

kilo = 1,000 of something

So moles are kind of the same thing – just larger, and with a number that, at first, seems odd:

mole = 602,200,000,000,000,000,000,000 of something

or

mole = 6.02 x 10 23

(Well, not exactly. It is really 602,214,179,300,000,000,000,000,
but we’ll round off.)

What does a mole of atoms look like? Here you can see a mole of aluminum atoms, copper atoms, and of carbon atoms.

Wow, even though there are billions of billions of billions of atoms, they still add up to small volumes. So each atom must be pretty small!

One Mole metal cubes

Image from acschemmatters-digital.org

Ok, I get that we might need a big number.

But instead of 6.02 x 10 23 why not make it simple,
like 10.0 x 10 23 ?

So how did the concept of the mole develop? Why is it defined the way that it is?

Visualizing large numbers, and how the mole idea developed

 

Mole was defined for practical purposes

Why are moles designed to be this number specifically? Jeremie Wainstain teaches us:

The definition of a mole comes from a practical need: how can I easily count atoms? This is a real need in Physics or Chemistry because at a molecular level, you really have, for instance, 1 molecule that interacts with 1 or 2 others, so you really need to “count” things, even if there are a lot.

So the brilliant idea of the concept of “mole” is to say that 1 mole of atoms = the atomic mass of the atom, but in grams.

Therefore

1 mole of carbon = 12 g of carbon

1 Mole of Oxygen = 16g

1 mole of Hydrogen = 1g

That’s brilliant because the atomic mass is easily accessible in the periodic table, so you can get it easily to make experiments.

And if you think about the concept of atomic mass, you will see that this definition correspond in fact to the same quantity of matter, the same number of atoms.

Same works with molecules :

A Mole of water = 1 Mole of Oxygen + 2 moles of Hydrogen = 18g of water

And this is again the same number of molecules.

It even gets easier: You usually don’t even need to know how many elements you have in a mole of elements to work with it.

It is approximately 6.10^23, but most of the time we don’t need to solve problems like that.

How do to molar math problems

Chemistry problems using moles isn’t as hard as one may think. It’s all about dimensional analysis, setting up fractions so thatunits cancel out.

Molar math

External references

The mole (lesson,) Fordham Prep

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