# Mole Concept

Under mass conversions, connections between grams, moles and molecules are established.

## The Mole

The mole is the chemical unit for the amount of a substance and serves as the bridge that connects grams to molecules, atoms, formula units and ions.

## Mass Conversions

Concept: Avogadro’s Number, Moles, Molecules & Atoms

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Before we begin, we should realize that mass conversions are going to play a very important and essential role in almost all the calculations we're going to do this semester.
We're going to start off by saying that the mole is the chemical unit for the amount of a substance. What exactly does this mean? Instead of a chemist ordering a hundred pounds of a particular element, a scientist could easily say 'I need 10.12 moles of carbon' or '100 moles of nitrogen'. It's basically a certain way that chemist communicate with each other to designate the amount, the weight they need of a particular element or compound.
We're going to say that one mole which you can write as this way, one mole contains 6.022x1023 entities. We're going to say that this special number has a name to it. This special number is known as Avogadro's number. When we say the word 'entities', entities means different things. When we say entities, entities can mean atoms, molecules or particles.
We're going to say we use atoms when we're dealing with a single or individual element. If I want 1,000 atoms of oxygen, I'm only talking about single oxygen. Remember oxygen can exists as a diatomic molecule, but here we're not talking about the molecule, we're talking about just the atom.
We're going to say we use molecules or particles when dealing with more than one element together or a compound, so just remember the difference. Atoms go with a single elements, molecules or particles goes with more than one element together or a compound. Compounds remember are just different elements combined together.

The words atoms and ions are used when talking about a single element and the words molecules and formula units are used when talking about more than one element together.

Concept: Atoms to Moles to Grams

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How does this relate to our calculations we're going to do this semester?
Here we have two good examples of when do we use atoms versus when do we use molecules or particles. If we take a look, we say we have 6.022x1023 atoms of iron. Iron is there by itself, so that's why we're using the term atoms. Remember that's Avogadro's number. That number is always equal to one mole of whatever we're talking about. Since we have this many atoms of iron, that means we have one mole of iron.
We're going to say also that one mole of something is related to its weight. It's atomic mass which you'll find on your periodic table. If you look at your periodic table right now, look for iron, it's a transition metal. One mole of iron is equal to its atomic mass that you see on your periodic table. If you take a look, you'll see 55.85. That's representative of its atomic mass unit or we can say 55.85 grams per mole.
What we need to realize here, that there's a connection. We can go from atoms to moles to grams. I show you double arrows meaning we can go either way. So we can go from atoms to moles or moles to atoms. We can go from moles to gram or grams to moles. The arrows go both ways.
What we should realize here is that moles play the connection that's between atoms and grams. Remember you can’t go straight from atoms to grams without having first go through moles.

Example: Determine the mass (in grams) found in 7.28 x 1028 nitrogen atoms.

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Concept: Molecules to Moles to Grams

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We've seen how atoms work. Now let's see how molecules work. We're going to start out again with Avogadro's number, but this time in terms of molecules of water. Remember that Avogadro's number is equal to one mole of whatever we're talking about, whether it be an atom, whether it be a molecule, whether it be particles, so that's equal to one mole of water.
One mole of any substance, whether it be an individual element or a compound, one mole of anything is equal to that objects molecular weight, its atomic mass. Here we have a mixture of different elements together, we have two hydrogens, we have one oxygen and water. To find out its total mass, we're going to add up each individual part.
We have two hydrogens, right? Because this little two here with the H, if you look on your periodic table, look for the atomic mass of hydrogen. When you look at it, you'll see that on your periodic table, its 1.008 grams and there are two hydrogens, so multiply that number times two. So that's 2.016 grams per mole.
Also, we have one oxygen which weighs 16 grams according to our periodic table, so that's 16 grams per mole. When you add those two numbers up together, that gives you the 18.016 grams per mole or 18.016 atomic mass units. That's where that number came from. We looked up each of the atomic masses of each individual element and then added them up together to get our answer.
Remember, Avogadro's number is always equal to one mole of whatever we're talking about. One mole of anything is just equal to the total mass of that object. If we take a look at this example, we're going to say it says, 'Determine how many molecules of carbon dioxide are found in 75 grams of carbon dioxide?'
Now, you should notice that this chart is similar to the one we just saw, except now instead of seeing atoms, we see molecules. You could also put particles in there too. Remember, there's a connection between molecules at moles, you can go from molecules to moles or moles to molecules and again there's a connection between moles and grams.

Example: Determine the how many molecules of carbon dioxide, CO2, are found in 75.0 g CO2.

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## Mass Conversion Calculations

Problem: If the density of water is 1.00 g/mL at 25oC. Calculate the number of water molecules found in 1.50 x 103 µL of water.

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Problem: Calculate the number of oxygen atoms found in 783.9 g in copper (II) sulfate pentahydrate, CuSO4 x 5 H2O.

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Problem: The density of the sun is 1.41 g/cm3 and its volume is 1.41 x 1027 m3. How many hydrogen molecules are in the sun if we assume all the mass is hydrogen gas?

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Problem: A cylindrical copper wire is used for the fences around a house. The copper wire has a diameter of 0.0750 in. How many copper atoms are found in 5.160 cm piece? The density of copper is 8.96 g/cm3. ( V = π x r2 x h )

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