# Gchem_Kaplan test #1/Q#26

Discussion in 'DAT Discussions' started by Glycogen, Jun 19, 2008.

1. ### Glycogen 2+ Year Member

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2X (g)+ Y(g) --> Z (g)

Ok,Q states that if stoichiometric quantities of X (g) and Y(g) are introduced into sealed,rigid container at constant tem with an initial pressure of 12 atm,what will the pressure in the container be when the reaction in complete?

a.12 atm
b.6 atm
c.4 atm
d.3 atm
e.2 atm

ok, I choose c simple b/c I thought when 3 mole=12 atm then 1 mole=4 atm
and apparently my reasoning was right BUT,then I was tempted to choose 12 atm simply b/c of wording,when it says sealed and rigid that means there is no interaction of gases with outside so I thought maybe the pressure will stay the same!!!
I mean I know that the first reasoning was right but I just want to know why they used sealed and rigid container there?does make any diffrence if we had open container!
Thank you.

2. ### harrygt 2+ Year Member

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Your first reasoning is correct as you said.
You are missing a point in the second reasoning. If the container is open, everything will diffuse out, and the pressure will always be equal to atmospheric pressure [1 atm]. whenever you have some gases reacting toghether, you should always seal the conainer, otherwise the gases will diffuse in the air.

3. ### Glycogen 2+ Year Member

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Ok,thank you and so what about rigidity!?
b/c there very next Q asked about flexible container and their approach was totally different.I've got a bit lost.

4. ### osimsDDS 5+ Year Member

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I think because they are stating that volume and temperature are constant but the moles are not because in order for the moles to be constant then volume, temperature, and pressure have to be constant I believe...

not 100% sure on this someone double check me...

5. ### Glycogen 2+ Year Member

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That's exactly where my confusion comes from.b/c it doesn't state in the Q about constant tem and pre but in the explanation says that.I don't know where that came from.How would I be able to know that the pressure and tem are constant?
Does have anything to do with flexibility of the container?

6. ### harrygt 2+ Year Member

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Ok, about rigidity, once the container is rigid, it means the volume can not change, therefore the only variables in the equation PV = nRT are n[number of moles in the container] and P. [Rigidity makes V constant. Temp is mentioned to be constant too]. In this question n dropped from 3 to 1, so pressure will drop from 12 to 4 to keep both sides of the equation equal.
Let me see the other question if you want.

7. ### Orgodox 7+ Year Member

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I know the next question you are talking about but check it out it is also the same logic you applied to this problem. 3 moles = 30 L then 1 Mole after reaction if complete has 10L
Also can think of it as PV=nRT and since Pressure is going to remain constant since it is sealed (see *) then when n decreases by 3X V must do the same because they are proportional.
* They say the pressure will remain constant. Why in this question do they say flexible and the previous question they say rigid? I understand why rigid in the last one but If it is flexible here then who said the pressure will remain the same??

8. ### osimsDDS 5+ Year Member

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"X (g) and Y(g) are introduced into sealed,rigid container at constant tem with an initial pressure of 12 atm"

This is telling you 3 things...sealed rigid container = constant volume
Constant Temperature = no temp change and its constant
Initial pressure = that means pressure will change...

These problems are tricky cuz you have to know the functions of what they are talking about...also you have to know all the laws...like avogadro's law for this one, guy lussac's law, boyles law...etc.

9. ### osimsDDS 5+ Year Member

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rigid is a volumetric function i believe....sealed is a pressure function...

When you have a rigid container it means it can not be expanded therefore the volume is constant...if they say its sealed it means that pressure is constant...

since they are saying it is flexible that means that volume can change and therefore it is not constant...and sealed tells you that pressure is constant...
so from PV=nRT you can cross out both P, R, and T (since they prob say at same temp)

10. ### Glycogen 2+ Year Member

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I know.They are playing with words and by doing that the whole concept can be changed.
Which is good,since the real DAT is all about concept!
Thank you guys.

11. ### Glycogen 2+ Year Member

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BTW,if any of you have the answer key to test # 3 of KAPLAN Gchem and would send it to me,I'll be appreciate it.
The one starts with Mn+ H2O --> MnO4- +H
Thanks.

12. ### Orgodox 7+ Year Member

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Good explination. I got that one but the next one doesnt make sene to me and also goes against what you explained which seems to make sense to me too. Here the quesiton:

13. ### Orgodox 7+ Year Member

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2X (g)+ Y(g) --> Z (g)

If stoichiometric quantities of X (g) and Y(g) are introduced into a SEALED, FLEXIBLE container with an initial volume of 30 liters at STP what volume of Z will be produced?

10 L
30L
90L
3.5 L
5L

If it is sealed how is volume changing?

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Must be 10L?

15. ### osimsDDS 5+ Year Member

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Watch this, this is EXACTLY how i reasoned this question and did it on paper...

SEALED means that the pressure isnt changing...therefore it is constant...

SEALED = no pressure change...pressure is constant
FLEXIBLE = volume can change...volume is NOT constant

therefore...PV=nRT you can cross of P, R, T (since its saying STP which is standard temperature) you are left with V=n

now use Avogadro's rule which states V1/n1=V2/n2

So you will get 30/3=V2/1
V2= 10 liters

16. ### Orgodox 7+ Year Member

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Nice! Thanks for clear explanation of both. I think it was even harder that they were back to back like that. But now I see how to translate those precise word choices. Tricky

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