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Topic: ideal gas law  (Read 4674 times)

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rose

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ideal gas law
« on: July 01, 2008, 03:48:30 PM »
Hi!
I have the following data concerning thiophene:
P = 0.047 atm (I did not find this value in a handbook, I calculated myself; if somebody have found the vapour pressure of thiophene at 10.8 C, I will highly appreciate if I will be informed)
T = 283.8 K
V = 0.00134 L
R = 8.314 J/K.mol (or 0.08206 atm.L/mole.K).
Having these values and considered the cas of the ideal gas, I determined n, accordingly to the formula:
PV = nRT.
I obtained 2.7 micromoles of thiophene. Is that correct? I am asking because I found in another source, a different value for n. I have to mention that I do not know the pressure value at 283.8 K for thiophene used in that source.
Thank you!

enahs

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Re: ideal gas law
« Reply #1 on: July 01, 2008, 03:59:02 PM »
Your math is correct, assuming you calculated the pressure correct.

How did you calculate the pressure though?

rose

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Re: ideal gas law
« Reply #2 on: July 01, 2008, 04:12:41 PM »
Thank you!

I found in the literature the following pressures for thiophene:

temp (C) (x)        P (mmHg) (y)
0      18
12.5      40
25      79.7

I put these values in a graphic and I obtained a polynomial trendline of order 2 with the following equation:
y = 0.0567x2 + 1.0514x + 18, where y = pressure, x = temperature (10.8 C)

Finally, I obtained P = 0.047 atm (When I use temp in K, I obtain 0.044 atm)

tamim83

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Re: ideal gas law
« Reply #3 on: July 02, 2008, 10:13:47 AM »
Use a linear trendline because a P vs T plot should be linear, they are directly proportional.  Also, you must use temperature in Kelvin for all gas law problems.

rose

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Re: ideal gas law
« Reply #4 on: July 02, 2008, 10:16:40 AM »
I used the polynomial trendline because it was the best fitting. In calculations I used temp in K!!!

Anyway, I think that I understood why they obtained a different value. So, as you saw, I obtained 2.7 micromoles while they obtained 59 micromoles because they used in the equation, not the thiophene vapor pressure at 10.8C but the standard conditions of T and P. So, in this case, the final value will be 59. But I am not sure that it is correct like this.
On the other side, if we dived he volume of thiophene (0.00134 L) to 22.4 L, we’ll get 59 micromoles, as well.

What actually happen?
We have a flow of thiophene in H2. This flow is 25.7 ml/min and contain 5.2 vol% of thiophene.
What do you think? Is OK to use STP conditions to calculate the number of mole of thiophene in this flow?