Have a question from GC...would appreciate your help!

[YourDentition]

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Hi, I wan't sure as to why the answer to the below question right. Could you help? This is from Topscore.

What is the final pressure of a gas that expands from 2L at 10(degree C) to 20L at 100(degree C), if the pressure was initially at 6 atm?

Answer: .79

I though that you divide temperatures (in K) to their respective pressures and make them equal one another....in order to find the Pressure change.
Why is this one different?

Would appreciate you help.
Thanks.

 

[nze82]

Hi, I wan't sure as to why the answer to the below question right. Could you help? This is from Topscore.

What is the final pressure of a gas that expands from 2L at 10(degree C) to 20L at 100(degree C), if the pressure was initially at 6 atm?

Answer: .79

I though that you divide temperatures (in K) to their respective pressures and make them equal one another....in order to find the Pressure change.
Why is this one different?

Would appreciate you help.
Thanks.

P1V1/T1 = P2V2/T2

P2 = P1V1T2/T1V2

You need to convert the temperatures to Kelvin before you can proceed:
K = C + 273
So:
T1 = 10 + 273 = 283K
T2 = 100 + 273 = 373K

P2 = (6atm)(2L)(373K)/(283K)(20L) = 0.79atm

 

[GiTsticker]

Question regarding the calculation -- How would you guys do that calculation with scratch paper quickly? I simplified to (3)(373)/(283)(5) then the rest was multiplication and long division which took me a while on the test. Will we be expected to actually do these calculations on the real test or will they be for the most part written in equation form?

To the OP, you forgot to take into account that the volume changed also.

Boyle's Law
P1xV1=P2xV2

and

Charles' Law
P1/V1=P2/V2

The best equation to remember is P1V1/n1T1 = P2V2/n2T2 because it takes into account changes in all factors that affect gases. If any factors don't change, then eliminate them from the equation.

 

[nze82]

Question regarding the calculation -- How would you guys do that calculation with scratch paper quickly? I simplified to (3)(373)/(283)(5) then the rest was multiplication and long division which took me a while on the test. Will we be expected to actually do these calculations on the real test or will they be for the most part written in equation form?
You'll get both types of questions. Sometimes the answers are posted in the equation form; sometimes, they're looking for a numerical answer.

To the OP, you forgot to take into account that the volume changed also.

Boyle's Law
P1xV1=P2xV2

and

Charles' Law
P1/V1=P2/V2

The best equation to remember is P1V1/n1T1 = P2V2/n2T2 because it takes into account changes in all factors that affect gases. If any factors don't change, then eliminate them from the equation.

So, it all comes down to whether or not you should spend all that time doing all the rigorous multiplications and divisions? I actually posted a comment about that on a different thread. Check it out:
http://forums.studentdoctor.net/showthread.php?t=637062

 

[DoctaWalnuts]

P1V1/T1 = P2V2/T2

P2 = P1V1T2/T1V2

You need to convert the temperatures to Kelvin before you can proceed:
K = C + 273
So:
T1 = 10 + 273 = 283K
T2 = 100 + 273 = 373K

P2 = (6atm)(2L)(373K)/(283K)(20L) = 0.79atm

I'm nominating you for SDNer of the year. 🙂