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Title: Molecules, Empirical Formulas,& Molecular Formulas
Description: otherwise known as confusing chemistry


AshleyG - February 12, 2003 02:15 AM (GMT)
Hey Guys,
I'm having troubles with my chemistry class that should really be labeled an independant study since the teacher is just there to make sure that April and I don't kill each other. Everything was okay and I was making sense of the book until we started this chapter complete with molecules, empirical formulas, and molecular formulars. I've tried to get the teacher to help me but he isn't much help. I'm wondering if anyone can explain this in terms that you don't have to have a masters in science to understand......
This are some sample problems:
Molecules- If one molecule of CO2 has a mass of 44 u, what is the mass of 6.02 x 10ēģ molecules of CO2? (the problem here is that the answer is supposed to be in grams.. :/ )
Empirical formula- What is the empirical formula of a compound that is 25.9% nitrogen and 74.1% oxygen? (I'm following the book but I'm getting a 40 to 1 ratio... :blink: )
Molecular formula- Symclosene.........has a empirical formula of CCINO and a molecular mass of 232.42 u. What is the molecular formula? (I have no idea how to even start to find this one! :unsure: )

BoBayles - February 12, 2003 03:01 AM (GMT)
I see Patrick's reading this topic at the same time I am, so I'm betting his explanation will trump mine, but here's some concepts.
The 6.02x10^23 is an important number. Avogadro was a perceptive individual; he recognized that if you have around 6.02x10^23 atoms of an element just happens to weigh about what its atomic mass is... in grams. 6.02x10^23 of an element or molecule is called a mole. For example, a mole of carbon atoms weighs 12.01 grams (look on your periodic table). A mole of oxygen atoms weighs 15.99 grams. A mole of carbon dioxide is composed of a C, and two O's. 12.01+15.99+15.99 is your answer for that, in grams. 43.99 grams is how much 6.02x10^23 molecules of carbon dioxide. Your book rounded the 43.99u of one molecule of CO2 from 43.99 to 44u... 6.02x10^23 molecules of CO2 weighs 44 grams.
Read this if you want some more info, but beware, it'll corrupt your belief in ideal gases and possibly confuse you more.
I've gotta go, if nobody else has helped, I'll give the rest a shot.

Seth - February 12, 2003 03:21 AM (GMT)
Lol that wasn't Patrick, it was me! I'm red too! :P

The whole "u" thing confuses me. It's not in any of my books. But Bo is right, 6 x 10^23 is a mole.

For percentage calculations, use this formula:
QUOTE (My chem textbook)
Mass percentage = Mass of C in sample divided by total mass of sample times 100%

or if you're given the percentages, just work backwards. I assume you've had Algebra II. If you keep having trouble, post back... I'll ask my chem teacher about the whole "u" thing :ph43r:

Polarris Delsan - February 12, 2003 03:22 AM (GMT)
hehe, it was me. This just took a long time to write, so I disappeared from the online list :P

ooooooo, fun stuff.
First of all, what do you mean by saying having things in units "u"? Is that just "units" or something?
EDIT: oh, I think it's AMUs, atomic mass units.
If I use dashes (--) in molecular formulae, it's just for clarity because I can't use subscript. (HTML is off :/)

Molecules - For starters, it's a good know that all the masses on the periodic table are in grams per mole. For instance, one mole of Carbon weighs about 12 g/mol. Also, 6.02 x 10ēģ is avagadro's constant, the number of molecules in a mole.
Look at your periodic table. You should see that Carbon has a mass of about 12 g/mol, and Oxygen has a mass of about 16 g/mol. One carbon, and two oxygen = 12 + 16 + 16 = 44 grams/mole. If you have one mole, then you have 44 grams. So 6.02 x 10ēģ molecules of carbon has a mass of 44 grams.
I don't know what the problem means by saying "a mass of 44 u". I guess it means that one mole of that substance would be or something.

I'll explain empirical formulas here; if you already know what they are, skip this paragraph.
The Empirical formula is like the smallest ingredient in the molecule. Suppose for instance, that we had an element Sock. Sock is a dimolecular element, so we only find it naturally as Sock2. After all, whoever heard of a single sock? (Besides a dryer :rofl: ) Even though the molecule is Sock2, the empirical formula is just Sock, because Sock2 is a multiple of Sock. The empirical formula is the simplest form of the molecule.
For example, glucose, C6-H12-06 has an empirical formula of CH20. That's the basic formula. Put in 6 of those, (CH20)6, and you get glucose.

First of all, you want to assume you have 100g of the compound. That is, you have 25.9 grams of Nitrogen, and 74.1 grams of Oxygen. If you want to compare the two, you have to have them in terms of moles because they weigh different amounts. Lets go through some stoichiometry.
You have:
25.9 grams of Nitrogen / (14.01 grams of Nitrogen/mol) = 1.85 mol nitrogen
74.1 grams of Oxygen / (16.00 grams of Oxygen/mol) = 4.63 mol oxygen
You get about 1.85 mol N, and 4.63 mol O. You can now compare the number of mols of the elements together, starting with the smallest.
There are only two here, so this will be easy. You have 4.63 mol O / 1.85 mol N which is about 2.5. So for every 1 Oxygen in the compound, there are about 2.5 Nitrogens. Multiply by two to get an even number, and you get 2 Oxygens for every 5 Nitrogens. Thus the empirical formula is O2N5. The ratio is 2 to 5.

This leads up to the next problem because, while the empirical formula can tell you what the compound is made up of, it isn't the actual formula. If you know the molecular mass, you can get the formula :)

This confused the hell out of me for a minute. I looked it up and found that the empirical formula is not CCINO, it is CClNO, that is Cl or clorine, not CI, or carbon iodide ;)
Ok, for starters, we have CClNO, which has a mass of 12.01 + 35.45 + 14.01 + 16.00 = 77.47. The only thing you have to do, is take the mass of the whole molecule and divide by the mass of the empirical formula. 232.42 / 77.47 is very close to 3. This means that you can put in 3 of the empirical formula. Put in three of each Carbon, Clorine, Nitrogen, and Oxygen to make the formula for Symclosene C3-Cl3-N3-O3, which is what it says on this page. If you check the mass of that, it should be 232.42.


I hope that helped, and arrived in time for you to use it. Good luck with Chem. :arr:

Polarris Delsan - February 12, 2003 03:50 AM (GMT)
hehe
You know what's even more fun to say? Phenolphthalein :D
Don't ask me why I know how to spell that.


Ok fine.
I found the word "tetrachlorophenolphthalein" in the chemistry bible and then went around saying it to everyone :P

It's even more fun to say phonetically: Feenol-fof-a-lath-ian! (well, not quite phonetically, but that's how my chemistry teacher said it when she was a student :rofl: )

AshleyG - February 12, 2003 06:33 PM (GMT)
You guys are a lifesaver!!!!!!!!! Thanks.

And yes little u is supposed to represent atomic mass units.

AshleyG - March 31, 2003 05:06 PM (GMT)
Thanks again guys, I managed to pull an A in chem this semster!

Polarris Delsan - March 31, 2003 07:34 PM (GMT)
Well done Ashley. Did you semester just end a little while ago??

AshleyG - March 31, 2003 08:58 PM (GMT)
Whoops, that should have said quarter not semster....we were talking about end of the semester grades while I was posting. lol

Polarris Delsan - April 1, 2003 12:51 AM (GMT)
Good, you scared me :lol:
Our quarter ended on Thursday, we're on our (very SHORT) spring break now :)

AshleyG - April 1, 2003 01:00 AM (GMT)
Ah, spring break that sounds so very nice...with all the snow days that we had I wouldn't have been surprised for our semster to just now be ending.




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