I don't quite understand the explanation for Q #3. I am unable to include the actual Q because of the graph that is given. I know this is a long shot but thought I would try asking anyways.
Thx!
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TPR FL practise test PS Q
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- Nov 19, 2006
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Sorry, it is from test #3.
Passage:
For any ideal gas, the number of moles, n, should be equal to the ratio PV/(RT). The graph below shows a plot of PV/(RT) vs. Pext for one mole of several real gases and an ideal gas. The Pext values on the horizontal axis are the external pressures at which the PV/(RT) ratios are calculated; they range from normal (around 1 atm) to very high (1000 atm). For one mole of an ideal gas, PV/(RT) = 1, regardless of the external pressure.
The PV/(RT) curve for one mole of methane (CH4) is typical of most real gases: it decreases to a minimum at moderately high pressures and then rises as pressure increases further. This curve shape is the result of the effects caused by the following characteristics of real molecules: at high pressure, values of PV/(RT)lower than ideal (that is, less than 1) are due predominantly to intermolecular attractions; values of PV/(RT)greater than ideal (that is, greater than 1) are due predominantly to molecular volume.
Q:
When carbon dioxide in a closed container is subjected to external pressures less than 650 atm, the deviation from ideality is primarily due to the fact that:
A. calculated gas pressure is less than actual gas pressure.
B. calculated volume is less than actual volume.
C. actual number of moles is greater than calculated number of moles.
Correct Answer
D. actual gas pressure is less than calculated gas pressure.
Passage:
For any ideal gas, the number of moles, n, should be equal to the ratio PV/(RT). The graph below shows a plot of PV/(RT) vs. Pext for one mole of several real gases and an ideal gas. The Pext values on the horizontal axis are the external pressures at which the PV/(RT) ratios are calculated; they range from normal (around 1 atm) to very high (1000 atm). For one mole of an ideal gas, PV/(RT) = 1, regardless of the external pressure.
The PV/(RT) curve for one mole of methane (CH4) is typical of most real gases: it decreases to a minimum at moderately high pressures and then rises as pressure increases further. This curve shape is the result of the effects caused by the following characteristics of real molecules: at high pressure, values of PV/(RT)lower than ideal (that is, less than 1) are due predominantly to intermolecular attractions; values of PV/(RT)greater than ideal (that is, greater than 1) are due predominantly to molecular volume.
Q:
When carbon dioxide in a closed container is subjected to external pressures less than 650 atm, the deviation from ideality is primarily due to the fact that:
A. calculated gas pressure is less than actual gas pressure.
B. calculated volume is less than actual volume.
C. actual number of moles is greater than calculated number of moles.
Correct Answer
D. actual gas pressure is less than calculated gas pressure.
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I don't know if I need to look at the graph but this seems to be the right answer. If the pressures are high, then the gas is not ideal... there will be intermolecular attractions. When there are intermolecular attractions the molecules (CO2) will be hitting the container less and therefore cause less pressure on the chamber. Therefore the actual gas pressure (with attractive forces) is less than calculated (ideal formula).
The non-ideal formula is (P + n^2a/V^2) (V-nb) = nRT
a increases w/ increasing attractiveness
b increases w/ increasing molecular size
So if nRT is constant and P + n^2a is proportional to nRT.... if n^2a increases, P must decrease
I'm guessing that the 650 atm is the deciding factor between moderately high and extremely high pressures. Therefore, if one were to use THAT deductive reasoning, I would have to say that the real pressure is less than the ideal because of the intermolecular force of attractions and that therefore D would be correct.
- edit - it appears me and bloody surgeon are on the same page...
So it's true; calc and actual are referring to real and idea.
You can look at the ideal gas equation that they give you (in rearranged form) to deduce this answer.
n=PV/RT ; R and T are fixed, and they said its a rigid container, so V also has to be fixed. So, if the PV/RT ratio is less than one then pressure has to be lower than predicted.
Pressure is lost because translational kinetic energy is lost to electrostatic potential energy between molecules.
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