# My turn with a chem question

Trying to calculate volume using VanDer Waal’s equation.

Neon 1.0 mole

500.0 atm

355.0 K

[P + (n2a/V2)](V - nb) = nRT

I just can’t seem to isolate volume in this equation and if we went over it in class I missed it as I spent most of the class holding my breath to avoid coughing spasms.

Honestly, we didn’t get a chance to do practice in class and he didn’t walk through things being he could not get the overhead to move out of the way so he could access the blackboard so he just moved on further in the lecture without explaining the equation.

I don’t need you to DO the problem, but if you can help me rearrange the variables so I can actually do this thing that would be nice.

I keep ending up cancelling all liters or all "V"s which can’t possibly be right.

Is this Chem 2? We haven’t gotten anywhere near this in Chem 1.

I’m working on Limiting Reagents, Combustion Reactions, and Percent Yield.

Yeah it’s chem 2 but not. It’s a catch up review of certain aspects that are in the early chapters of the book. In chem 1 we skipped it. I’m not sure if they did in chem 1 at this school though, so I could just be dumb.

http://en.wikipedia.org/wiki/Van_der_Waals_eq uatio…

You might be able to take what Wiki has here with the numbers given. Looks like there are some constants that you can plug in in order to solve for volume.

Yes there are constants, but with the different aspects of the constants I’m not figuring out how to cancel everything. I saw the wiki version and another solving for volume but R ended up being to the 3rd power and I ended up real confused. Oh wait, I already am confused. Nevermind. hehe.

I do have the constants though. They are provided in a table. One is moles squared times atmospheric pressure all over volume squared

http://hyperphysics.phy-astr.gsu.edu/Hbase/Kinetic…

Did you check this one yet?

Volume is always measured in cubic somethings. So no worry to be confused if your answer is to the 3 power.

Dude that just made my eyes glaze over. Hehe.

It has constants with moles to the 3rd power? My book has no such monster. And it’s not a monster with Hugz either. I’m just sayin’.

Still looking. Thanks for the valiant attempts at chivalry tonight Hugz. I’m hoping someone who’s gotten through this class already can help. Perhaps I can return the favor with chem 1. I managed pretty good in that class after hireing a private tutor. Things just started making sense then. Must. Find. Tutor.

Yeah, that link looks to be an advanced physics site. I’ll stick to Chem 1 and leave the Chem 2 business to a more seasoned student.

• BOOBS Said:
Trying to calculate volume using VanDer Waal's equation.

Neon 1.0 mole

500.0 atm

355.0 K

[P + (n2a/V2)](V - nb) = nRT

I just can't seem to isolate volume in this equation and if we went over it in class I missed it as I spent most of the class holding my breath to avoid coughing spasms.

Honestly, we didn't get a chance to do practice in class and he didn't walk through things being he could not get the overhead to move out of the way so he could access the blackboard so he just moved on further in the lecture without explaining the equation.

I don't need you to DO the problem, but if you can help me rearrange the variables so I can actually do this thing that would be nice.

I keep ending up cancelling all liters or all "V"s which can't possibly be right.

Well, the way you expressed it is a little unclear as far as the mathematical notation so with the stuff in brackets, does it all have to stay together like that or not? Can it simply be P + (n2a/v2)(v-nb) = nRT?

If so, then your problem is somewhat straightforward since you do have some values for variables. n is simply 1 since you're doing with 1 mol so you can just omit n altogether since it doesn't change the equation, T is a known variable so you can plug that in, R is a constant which you have, and P is a known variable so you can plug that in. Once you plug all that stuff in, the only variables left should be a and v....just get a on one side, v on the other, and you're golden. You first have to clarify whether it's V2 (which makes no sense since then you have two volume variables) or 2V. (:

sorry the 2’s didn’t copy paste from the online formula. I was trying to get one that had the actual powers raised. The formula in our text is

[P+a(n/v)^2]*[V-nb]=nRT

the a and the b both have values much like R and getting everything to cancel is throwing me foreloop.

I keep ending with V=(insert weird incorrect number here) L^2/mol

• BOOBS Said:
sorry the 2's didn't copy paste from the online formula. I was trying to get one that had the actual powers raised. The formula in our text is

[P+a(n/v)^2]*[V-nb]=nRT

the a and the b both have values much like R and getting everything to cancel is throwing me foreloop.

I keep ending with V=(insert weird incorrect number here) L^2/mol

Honestly I'd have to see the math equation exactly as it's shown in the book along with the given values to see what needs to be done. I always used www.mathhelpforum.com when I had a math question. If you can render the equation properly, scanning it and providing a picture would work best, chances are you will get several excellent responses on that forum as to what needs to be done. Alternatively, I would take this problem to the tutoring center or whatever you have at that university.

I figured it out. There is no need to account for inter molecular forces in this problem. As a result, I was using more of the equation than was needed.

I’m not even sure this is due tomorrow, but I figured better safe than sorry. The syllabus has this material scheduled to be covered tomorrow which means the assignment is due the next class, however, with him moving ahead in his lecture, I didn’t want to take the chance of not having it completed.

Thanks for all the help guys! Talking it through actually helped.

In Chem 1 and/or 2 VdW eq is just to quantitatively explain that gases deviate from ideal behavior at low temps/high press.

For calculations, typically PV=nRT is used. If you have to use VdW’s eq the question will hint at high press/low temp or specify the eq to use.

As far as the orig question - If you have

(a+b)(c-d) = x -> (c-d) = x/(a+b) -> c = [x/(a+b)]+d

If you’re having trouble with this then come up to speed on your algebra - you’ll need it quite a bit when you get to stoichiometry and especially for equilibrium calculations.

It’s not so simple as that. If you notice, there is more than one V term in the van der Waals equation. You end up with a cubic equation in V to solve, namely:

p(V^3)-n(RT+pb)(V^2)+(n^2 )aV-(n^3)ab=0

Not exactly the most straightforward to solve, unless you come across a degenerate case. That said, I agree with above - I remember using the ideal gas law much more than van der Waals for actual problems. pV=nRT is so much nicer to work with.

Just to note, it has been since high school since chemistry for me, so remembering the applications of each case is a bit fuzzy.

This one did specify to calculate using both PV=nRT and VDW then explain the difference in the results.

• BOOBS Said:
This one did specify to calculate using both PV=nRT and VDW then explain the difference in the results.

Good ol' piv-nert. I remember that equation. Man, why couldn't the rest of the equations in gen chem be as simple as the gas laws!?