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Yoshi Dude

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Posted on 10-11-04 08:47 AM Link | Quote
I need help on some Chemistry problems I have to do. I wont throw out every problem to you guys, just a few. I'm real confused. I showed my mom and she said moles was the reason why she didn't do well in the class herself.

Stuff to know (we round the numbers in my class)

6x10^23 particles/mole
Ne: 20.2g molar mass
Osub2: 32.0g molar mass


I don't know if molar mass adds anything to these problems.

How many moles in 5.2 x 10^23 atoms of Ne?
How many atoms in 3 moles of Osub2 molecules?

Now I don't just want the answer, I can get that, I need to know how to find it. I don't understand the relationship between grams, moles, particles, molecules, or atoms. Like, I know there's some formula that will let me do all of these problems but I just don't know what it is. I figure if I know how to do those two problems, I can do the rest.
I really appreciate any efforts, I'm barely getting an A in the class and if I get these wrong it'll shoot me down for sure.
hhallahh

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Posted on 10-11-04 09:03 AM Link | Quote
The molar mass of an atom/molecule refers to how much 6.02*10^23 units of that compound weighs, in grams.

For the first question, you have to divide Avagadro's (sp?) number (6.02*10^23) by 5.2*10^23.. think of it as if I asked you, "If there are 100 centimeters in a meter, how many meters is 35 centimeters?" It's a simple conversion, and you should be able to do that kind of question easy. I recall in Chemistry they made us write it out as:

((5.2*10^23 atoms Ne) / 1) * ((1 mol Ne) / (6.02*10^23 atoms Ne)) = ?

The first part of the equation simply writes out what you're starting with, 5.2*10^23 atoms Ne. The second part is a conversion. 1 mol Ne = 6.02*10^23 atoms Ne by definition, so you multiply your starting point by that conversion in order to get the answer. That might not be sufficiently clear, but it's hard to show without writing it out on binder paper..

The second equation's conversion can be written out as:

((3 mol O2) / 1) * ((6.02*10^23 molecules O2) / (1 mol O2)) * (2 atoms O / 1 molecule O2) = ?

Again, note that the first part of the equation is simply your starting point. The second part is the conversion from mols O2 to molcules O2... 1 mol O2 = 6.02*10^23 molecules O2. Also note that since there are 2 Os in every O2, another conversion of 2 atoms O2 = 1 mol O2 is needed to derive the total number of O. All conversions will have this format.. to go back to the simple example I used above:

((35 centimeters) / 1) * ((1 meter) / (100 centimeters)) = .35 meters

The first part is my starting point, the second is the conversion of 100 cm = 1 m. See the pattern?

(Geez.. so many edits. I'm sloppy about this.)

(edited by hhallahh on 10-11-04 12:04 AM)
(edited by hhallahh on 10-11-04 12:04 AM)
(edited by hhallahh on 10-11-04 12:06 AM)
(edited by hhallahh on 10-11-04 12:11 AM)
(edited by hhallahh on 10-11-04 12:11 AM)
Arwon

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Posted on 10-11-04 09:04 AM Link | Quote
It's been a few years since I did chemistry but let's see what I remember:

A mole is just a particular number. Like "a dozen" is 12, "a thousand" is 1000... "a mole" is 6x10^23.

So Ne, having a "molar mass" of 20.2 grams, that means that 6x10^23 atoms of Ne weighs 20.2 grams.

O2, meanwhile, has a molar mass of 32 grams, meaning that 6x10^23 molecules of O2 weighs 32 grams.

It doesn't make a bit of difference whether it's atoms or molecules - you can have a mole of either. It just depends on whether the substance is an element made up of single atoms, or a compound made up of molecules.

Moles are just a way to count particles, it doesn't matter whether those particles are single atoms or molecules.



For the first question it looks like you just have to figure out what percentage of 6x10^23 the number given (5.2x10^23) is... the answer should be something a little under 1 mole.

For the second, you need to first figure out how many atoms are in ONE mole of O2. The key is that they want atoms, not molecules. Think about what O2 is. It's a molecule consisting of two oxygen atoms. So in 1 mole of O2 there is 6x10^23 molecules, meaning there is TWICE as many atoms. So how many atoms in 3 moles of the stuff?

(edited by Arwon on 10-11-04 12:05 AM)
Yoshi Dude

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Posted on 10-11-04 09:51 AM Link | Quote
Thanks you guys.

Okay, so for the first problem 5.2*10^23/6*10^23 = .87 mole. Yeah?

The second one I'm a bit more confused on. So a mole of O2 molecule has 12x10^23 atoms in it, twice as much?
So that would be (12x10^23)3 = 36x10^23 atoms

Yes?
hhallahh

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Posted on 10-11-04 09:55 AM Link | Quote
Originally posted by Yoshi Dude
Thanks you guys.

Okay, so for the first problem 5.2*10^23/6*10^23 = .87 mole. Yeah?

The second one I'm a bit more confused on. So a mole of O2 molecule has 12x10^23 atoms in it, twice as much?
So that would be (12x10^23)3 = 36x10^23 atoms

Yes?


Yea, that's right. I don't know if you're learning the same conversion format that I did, but the way I write out the problems is very simple and if you can follow it, then you should be able to do any conversion your teacher could throw at you.
Arwon

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Posted on 10-11-04 10:16 AM Link | Quote
I feel it's more important to understand what you're doing than just learn conversion formats - they can come later. The danger with just learning formulas is inflexibility and inability to approach an unusual problem logically.

Understanding what a mole is - an arbitrary number of particles chosen to make it possible to mathematically deal with particle weights in the macro world (you can't see 1 atom and compare it with another type of atom, but you can see 6x10^23 of them and compare them to 6x10^23 of another type) - is also important.
NSNick
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Posted on 10-11-04 04:07 PM Link | Quote
It's also helpful to use unit analysis. If after the units cancel out, you have the unit you're supposed to, that's a good sign.

For example, in that first problem:

How many moles in 5.2 x 10^23 atoms of Ne?

5.2 x 1023 atoms   x       1 mol       =   .87 mol
                       6 x 1023 atoms

The atoms cancel to give you moles, which is what you're looking for.
kiwibonga

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Posted on 10-12-04 03:47 AM Link | Quote
I got through most of high school chemistry and physics without learning formulas... They give you 3 variables and tell you the unit the answer is in, that problem is another one of those where you can figure out the formula just by knowing the units.

Molar mass => Molar = mol, mass = grams ; since it's molar mass, and not... massic? massal? wtf? anyways, not the other way around, you figure out that it's grams in functions of moles, in other words, g/mol, and so all you have to do is mass/moles, etc

It's actually good to know that a Watt is actually a J/s, you know, that kind of stuff, once you get the hang of that you'll be able to slack off and not learn all this crap by heart Although it probably doesn't affect you guys as much... There's no end of year exams from what I could gather :/

Oh yeah and isn't that 6*10^23 usually referred to as Avogadro's number? (NA)
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Posted on 10-12-04 03:52 AM Link | Quote
god, do I NOT envy you.

I hope your teacher was cooler than mine. Hell I KNOW that he/she is, because I have horror/comedy stories about my chem teacher that make me shudder.
Colleen
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Posted on 10-12-04 08:29 AM Link | Quote
Ugh... don't remind me about chemistry. I could muddle through the formulas (that I can't remember now) all right but I was clueless in the lab.

The teacher was OK but it was still bleh. Problem was I was forced (since I was in the IB program) to take Chem or Physics.
NSNick
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Posted on 10-12-04 02:25 PM Link | Quote
Originally posted by kiwibonga
Oh yeah and isn't that 6*10^23 usually referred to as Avogadro's number? (NA)
Yeah. I think it's somewhere along the line of 6.04 x 1023
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Posted on 10-12-04 03:30 PM Link | Quote
What's the one where you have to know Carbon-12 as a constant and then the weight of each element to figure out a formula...is that molecular mass? I think I mixed that up with this one (the easier one) when I was trying to remember how to do Chemistry from four years ago for Ash. :\
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Posted on 10-12-04 04:24 PM Link | Quote
Originally posted by NSNick
Originally posted by kiwibonga
Oh yeah and isn't that 6*10^23 usually referred to as Avogadro's number? (NA)
Yeah. I think it's somewhere along the line of 6.04 x 1023


6.02 x 1023 if I recall correctly, though 6 is a good enough approximatoin.

BTW, Arwon, your first line of advice:

A mole is just a particular number. Like "a dozen" is 12, "a thousand" is 1000... "a mole" is 6x10^23.

...is what carried me through that part of chemistry. The concept of a "mole" was prety confusing until I started thinking about it this way.
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Posted on 10-12-04 11:06 PM Link | Quote
At first I had no idea what I was doing with Moles...then my teacher made a recipe analogy and it all clicked...after that I understood it perfectly, but from that point on everything just got more complex.
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