the dreaded van der waals equation

[leathersofa]

so i have been looking at multiple threads regarding the van der waals deviations but i am still really confused.

What is Vreal and Videal in the following equation: Videal = Vcontainer - nb? Would Vcontainer be the Vreal?

Also, how do we know whether the question is asking for Vreal or Videal? For example, in question 6.8 in TBR: For an inert real gas, if you were to reduce the pressure to half of its original value, then what is the final volume (Vf) relative to the initial volume (Vi)

shouldn't the answer be 2Vi + a little bit? because the equation rearranged is Vcontainer/real = Videal + nb
Therefore, 2Vi + a little bit = Vreal...also how do i know whether this question is asking for Vreal or Videal?

because each component would result in separate answers...

can someone please explain this in a simplified and straight forward manner? thank you very very much!!!!

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[onedirection]

There is no Videa or Vreal in the equation

it's just if the constants were zero it would equivalent to the ideal gas law

Assume it's ideal always. Because even real gases behave very ideally

The only reason it would act as a real gases if there are rather strong attractive forces

 

[leathersofa]

Assume it's ideal always. Because even real gases behave very ideally

The only reason it would act as a real gases if there are rather strong attractive forces

So for the question I posted (example 6.8 in TBR), i should always assume its ideal?

but the equation is clearly labeled as Videal = Vcontainer - nb
so i should just ignore the labels?

 

[onedirection]

So for the question I posted (example 6.8 in TBR), i should always assume its ideal?

but the equation is clearly labeled as Videal = Vcontainer - nb
so i should just ignore the labels?

I'll go check in the book give me one second


So start off
P and inversely proprtional to V

So 1/2 Pressure --> 2V

But you have to subtract out the volume taken up by the molecules themselves because it is a real gas, therefore it's not perfectly a doubling

The way I think about it, if the volume of the container is really small the gas particles get a little bit bigger than what the ideal gas law says because technically as the volume approaches 0, it would be impossible because the gas partilcles have an inherent volume; so the volume is larger than what the ideal gas law says

So the opposite case is true with an increase in side, and the volume is lower than what is predicted by the ideal gas law

 

[leathersofa]

wow, thank you so much. i really appreciate that 🙂 thank you!

 

[FeinMS]

Just remember that the change is always less than the expected. If you expect V->2V, the change is going to be less than that so 2V- a little. If you expect V->0.5V, the change is going to be less than that so 0.5V+a little

 

[leathersofa]

If you expect V->0.5V, the change is going to be less than that so 0.5V+a little

0.5V + a little means that the change is greater than 0.5V..would you please clarify a bit more? thank you!

 

[FeinMS]

0.5V + a little means that the change is greater than 0.5V..would you please clarify a bit more? thank you!

No 0.5V+a little means the change is less than 0.5...
V-0.5V=0.5V
V-(0.5V+a little)=0.499999 or something.

 

[leathersofa]

ohhh okay, thank you so much you two! i get it now, it's teh CHANGE! thank you!!!