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Offline manjui

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Chemistry homework help
« on: April 19, 2009, 10:41:28 AM »
Hi there. I am not looking for the answer to my homework, just help with understanding the problem so that I can figure out the answers.
Here is the problem: A neon sign is made of glass tubing whose inside diameter is 2.5 cm and whose length is 5.5 m. If the sign contains neon at a pressure of 1.75 torr at 31 C, how many grams of neon are in the sign?

Offline ARGOS++

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Re: Chemistry homework help
« Reply #1 on: April 19, 2009, 10:58:30 AM »

Dear manjui;

Use the Ideal Gas Law (IGL):   http://en.wikipedia.org/wiki/Ideal_gas_law 
(Select the adequate R for your problem.)

Finally you can convert the n from the IGL into grams.

Good Luck!
                    ARGOS++

Offline manjui

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Re: Chemistry homework help
« Reply #2 on: April 19, 2009, 11:15:21 AM »
Thank you ARGOS++.
One more question. In figuring out the volume since it is a tubing would I convert the diameter into cm or the length into meters?
This is the equation I came up with, if you could let me know if it is correct:
3.14 * 2.5^2 cm * 550 cm = 10793.75 cm.

Offline sjb

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Re: Chemistry homework help
« Reply #3 on: April 19, 2009, 11:18:08 AM »
It doesn't really matter whether you convert the diameter into m or the length into cm, as long as you're consistent, in my opinion. Looks like your numerical value is correct, but check your units.

Offline ARGOS++

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Re: Chemistry homework help
« Reply #4 on: April 19, 2009, 11:27:07 AM »

Dear manjui;

Your idea is correct!,  -  But the performance is wrong!     Sorry sjb!

The formula is: A =  r2 * pi * l  and not A = d2 * pi * l.
And take care for all Units!

Good Luck!
                    ARGOS++

Offline manjui

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Re: Chemistry homework help
« Reply #5 on: April 19, 2009, 11:33:03 AM »
Ok, that makes sense. Now I'm stumped as how to convert meters to grams. ???

Offline ARGOS++

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Re: Chemistry homework help
« Reply #6 on: April 19, 2009, 11:39:33 AM »
Dear manjui;

How you get the volume in meters??
Please show us your calculation!!

And remember:   And take care for all Units!

Good Luck!
                    ARGOS++

Offline manjui

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Re: Chemistry homework help
« Reply #7 on: April 19, 2009, 11:47:04 AM »
 ::) ok,
I converted 2.5 cm to meters getting 0.025 meters.
Then I figured the volume of the cylinder as: (1.25*10^-2 cm)(pi)(5.5 m) = 2.7*10^-3
Using the ideal-gas equation I converted pressure to atm (1330), and Celsius to Kelvin (304.15).
The following equation is what I did: (2.7*10^-3 m)(1330 atm)/(R 0.08206)(304.15)


Offline ARGOS++

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Re: Chemistry homework help
« Reply #8 on: April 19, 2009, 12:02:52 PM »

Dear manjui;

Your n is incorrect, because you did the pressure conversion wrong!

Your volume (= 2.6997 Liter) is correct, but you forgot again the units for!
Your temperature conversion (= 304.15K) is also ok!
Why not use the adequate R?   Use R = 62.363 L Torr K-1 mol-1

Now solve the IGL for n as next an show your result again.

Good Luck!
                    ARGOS++

Offline manjui

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Re: Chemistry homework help
« Reply #9 on: April 19, 2009, 12:35:25 PM »
I really appreciate you taking the time to help me with this!
Ok, I haven't see the R value of 62.363 L Torr K^-1 mol^-1 before in my chemistry book but I will definitely use it! I am assuming since I am using that value for R that I do not need to convert the 1.75 torr to atm? Or do I?
And 2.6997 L would be 2699.7 g, right?
Based on that here's what I did: 2699.7 g * 1.75 Torr/62.363 L Torr K^-1 * 304.15 K. That still isn't correct though....

Offline ARGOS++

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Re: Chemistry homework help
« Reply #10 on: April 19, 2009, 12:44:31 PM »

Dear manjui;

You are coming closer!

But  2.6997 Liter is not 2.6997g; there is no conversion necessary!

Your equation is correct, if you complet it for  n = pv / RT.
So the question is:  What is the unit for n, and its value?

(For R see the link I gave you!)

Good Luck!
                    ARGOS++

Offline manjui

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Re: Chemistry homework help
« Reply #11 on: April 19, 2009, 12:54:29 PM »
2.6997 L * 1.75 Torr/62.363 L Torr K^1- mol^-1 * 304.15 K = 2.49 * 10^-4

Offline ARGOS++

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Re: Chemistry homework help
« Reply #12 on: April 19, 2009, 12:57:36 PM »

Dear manjui;

Bravo!
It will be perfect, if you give me the unit for n too!

Good Luck!
                    ARGOS++

Offline manjui

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Re: Chemistry homework help
« Reply #13 on: April 19, 2009, 12:59:54 PM »
 ??? n?

Offline manjui

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Re: Chemistry homework help
« Reply #14 on: April 19, 2009, 01:00:56 PM »
I thought n was 2.49*10^-4

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