Chemistry Gases MDCAT 2017
PMDC Verified Question 59 of 65
In the equation \( \left(P + \frac{n^2 a}{V^2}\right)(V - nb) = RT \), 'b' represents the:
A
Excluded volume
B
Actual volume
C
Excluded pressure
D
Excluded volume per mole
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Propolis Cognitive Error Autopsy

Official Correct Choice:
Option D: Excluded volume per mole
Concept:

The equation provided is the van der Waals equation for real gases, which corrects the ideal gas law for the volume occupied by gas molecules and the attractive forces between them.

Solution:

  • The term \( (V - nb) \) accounts for the finite volume of the gas molecules.


  • The constant b specifically represents the excluded volume per mole of the gas (or co-volume), which is the effective volume that one mole of gas molecules prevents other molecules from occupying.


  • Therefore, \( nb \) is the total excluded volume for n moles.


Why other options are incorrect:

  • Option A: "Excluded volume" usually refers to the total excluded volume (\( nb \)), whereas 'b' alone is the excluded volume per mole.
  • Option B: 'b' is approximately four times the actual physical volume of the molecules, not the actual volume itself.
  • Option C: The pressure correction term is governed by the constant 'a', not 'b'.

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