TBR CBT 6 PS Question #7

[MedChallenge]

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Could someone explain why statement I is incorrect? I'm having trouble understanding why "Vobs" would be the real volume and not the ideal volume that is adjusted by subtraction of "nb". Also, how can III be correct and I be incorrect?



7. Which of the following statements is/are valid?

I. The ideal volume is greater than the real volume.
II. As the value of the b term increases, the size of the molecule decreases.
III. An ideal gas has both the a and b terms equal to zero.

A. I only
B. I and III only
C. II and III only
D. III only
D is the best answer. To obtain the ideal volume, the nb term (representing the molecule’s collective volume) is subtracted from the real volume. This means that the real volume is greater than the ideal volume, hence option I is false. Table 1 shows that as the atom (or molecule) becomes larger, the value for the b correction term is increasing. This is logical, considering that the correction for molecular volume must be larger as the molecule gets larger. This makes option II a false statement. If the a and b terms are equal to 0, then the correction terms in the van der Waals equation drop out, which then reduces the equation to PV = nRT, the ideal gas equation. If the value of a and b are equal to 0, then the gas must be ideal. This makes option III alone valid. The best answer is D.

 

[feipar]

The observed volume includes the volume of the gas molecules. The real volume (w/o molecular volume) would be smaller.

 

[MedChallenge]

The observed volume includes the volume of the gas molecules. The real volume (w/o molecular volume) would be smaller.

So the observed volume is the same as the ideal volume as I originally thought? If not, what is the relationship between the observed volume and the ideal/real volumes? The word "observed" makes me think it's the real volume, but statement III seems to contradict that.

 

[attixx]

These equations are correcting variables so they can be used in the ideal gas equation. Remember, and ideal gas has no volume taken up by its molecules.

In the equation: Videal = Vobserved - nb.

Videal can be thought of as free space. Vobserved is the actual, real volume of the container that you are observing; for example a balloon you measure to be 2 liters.

In ideal conditions, ideal volume is just the observed volume because the molecules take up no volume and the volume you observed is essentially all free space that you can manipulate by changing variables in the ideal gas equation (P, V, T, etc.)

However in reality, that 2 liters you measured cannot be used in the ideal gas equation because some of that volume is due to molecular volume. You only want the free space, the ideal volume that you can work with; and for real gases, some of that volume you measured is not free space but space taken up by molecules.

You subtract away the molecular volume from the volume you observed (real volume), leaving you with the free space; it is only that free space that can be used in PV=nRT to determine ideal gas relationships.


The real volume you observe will be GREATER than the ideal volume where molecular volume does not come into play.

 

[Rabolisk]

No, the observed volume is the real volume, and statement III does not contradict that. Statement III says that when a and b are 0, the Van der Waal's equation simplifies to the ideal gas equation. feipar is mistaken when he says that the real volume would be smaller than the observed volume. The real and the observed are the same, and larger than the ideal volume. Think about it, the only way one can define a "real" volume is to set it equal to observed volume. Otherwise, what is "real" about this volume?

For a more detailed discussion of such issues, consider this link.

http://forums.studentdoctor.net/showthread.php?t=804701

 

[MedChallenge]

Great explanations. Thanks guys.