Saturday, November 10, 2018

CHM 110 Useful Stuff

I'm taking CHM 110 at Trident Technical College, and I'm uploading documents to share with my fellow students and including useful stuff. This is purely for my college class. I'm going to keep bumping it to the top to make it easy to find.

Stoichiometry:

Stoichiometry is made more complicated than it needs to be for three reasons. First, the name. Stoichiometry just SOUNDS complicated. Easy solution, don't worry about what it's called. Second and third are harder and are actually the reason it exists at all. Stoichiometry involves things that are too small to see, and too numerous to count. If we could see and count atoms and molecules, stoichiometry would seem as easy as legos, probably even easier (most times your not trying to build the Millenium Falcon and often your just building with two or three different bricks - not so much in bio-chem - but this is 110!)

If I could grab a Chlorine atom and a Sodium atom and smash them together to make salt. Stoichiometry would seem like child's play. I think the key to UNDERSTANDING stoichiometry is to connect it to something you can see. I'm working on a comprehensive example of that which I will publish later. Right now, I'm just going to lay out some simple points to help you grasp what's going on.

First, the grab an atom of Sodium and grab an atom of Chlorine example, while simplistic, is pretty much how it works. To make salt, you need one Chloride atom and one Sodium atom. Na + Cl = NaCl. To make Barium Fluoride, you need 2 Fluorine atoms for every Barium atom. Ba + 2F = BaF2. It's really that simple, except that we need billions upon billions of atoms to make any appreciable amount of a substance.

So, since we can't see the atoms, scientists had to come up with a way to "count" the atoms. They needed some system that could convert something easy to measure into a number of atoms. They came up with a "mole". You can weigh a pure atomic substance, and based on what it is, know exactly how many atoms are in it (or close enough for chemical work - since atoms are so small and you can't see them - a few stray atoms here and there are usually not a big deal).

Scientists know that (using round numbers) 12 grams of Carbon will contain 6.02 x 10^23 atoms. They know this just as we know that 756 grams of large eggs will be 12 eggs or 156 grams of paperclips will be 144 paper clips (I checked). 12 eggs is a "dozen" eggs and 144 paper clips is a "gross" of paper clips. The "mole" is just a way of referring to a specific quantity of something, like a dozen eggs, or a gross of paper clips and using weight to "count" them. A mole is a term referring to a specific number, just like a dozen and a gross. With a little extra work, we can count things using their weight. It's not really useful for eggs in the kitchen, but if I needed to count 1000 eggs, or 10,000 paper clips, knowing that 144 paper clips weighed 156 grams is hugely important. With atoms, it's impossible to count them, so we have the mole.

We use moles for different substances, so we have different weights that equal a mole of them, just like a gross of different items would weigh a different amount.

One "gross" of paperclips would be 144 paper clips and would weigh 156 grams.
One "gross" of eggs would be 144 eggs and would weigh 9072 grams.
One "mole" of eggs would be 6.02 x 10^23 eggs and would weigh 3.79 x 10^25 grams.

So think of moles as kind of like bags of atoms or molecules. A bag of  Carbon atoms with 6.02 x 10^23 atoms in it weighs 12 grams. EVERY bag of atoms or molecules will ALWAYS have 6.02 x 10^23 of them in it, so all we need to know is how much a bag of all the substances weighs.

It's critical to be able to take an atom or a molecule and figure out its molecular or formula weight. That's simply figuring out how much a "bag" of the atoms weighs (our bag being a mole). For an atom, it's the atomic weight of the atom from the periodic table.

One bag (mole) of Sodium (Na) atoms contains 6.02 x 10^23 atoms (this number never changes - just like a gross of paper clips will always contain 144 paper clips) and weighs 23 grams (just like that gross of paper clips weighs 156 grams). It's a property of the substance and never changes.

One bag (mole) of NaCl molecules, contains 6.02 x 10^23 NaCl molecules and weighs about 58.5 grams (The atomic weight of Na (23) plus the atomic weight of Cl (35.5).)

The Atomic Weight or Formula Weight can be written as a ratio 1 mole Na / 23 grams Na, just like we can write for paper clips, 1 gross paper clips / 156 grams paper clips. These can be written upside down as needed to convert grams to moles or moles to grams, just like if I know how many paperclips I have I can use the ratio to figure out what they weigh, or if I know how much they weigh, I can figure out how many I have.

For formulas, the numbers in front of the molecules tell you how many bags (moles) of each you need for each of the ingredients, and how many bags (moles) of the various products you get.

Ba + 2F = BaF2 tells me that I need 2 bags of F for every bag of Ba and when I smush them together I get a bag of BaF2. Each of those bags has 6.02 x 10^23 atoms or molecules in it, and I can figure out how much each one weighs by figuring out their Formula Weights from the periodic table.

For calculation problems, convert grams or atoms to moles (always go through moles) and ratio them according to the numbers from the equation. Thinking of them as bags will help you remember that the term mole isn't magic, it just makes sure you have 2 atoms of F for every atom of Ba in the above formula. You are just using moles because there are a shit ton of atoms involved. I honestly believe that if we made the term "shit-ton" refer to 6.02 x 10^23 of something, and applied it to Chemistry, that everyone would find this a lot easier.

So this probably didn't teach you HOW to do Stoichiometry, but hopefully, it demystified it a little bit.

Keep checking back for updated information and suggestions.
Also feel free to use the Amazon link (the black and white tax book picture) to buy things from Amazon (from which I get a cut) or to buy my tax books.

Unit 1 Test Prep:

Click here for my Test Prep Notes
Click here for the marked up Periodic Table

Keep checking back for updated information and suggestions.
Also feel free to use the Amazon link (the black and white tax book picture) to buy things from Amazon (from which I get a cut) or to buy my tax books.

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