Question

Van der Waal $ ' $ s constants $ ' $ $ a $ $ ' $ and $ ' $ $ b $ $ ' $ related to _______________ respectively.
(A) attractive force and bond energy of molecules
(B) volume and repulsive force of molecules
(C) shape and repulsive force of molecules
(D) attractive force and volume of molecules

Answer 1

Hint: The constants $ a $ and $ b $ are called as van der waals constants. They are the correction factors for pressure and volume in the ideal gas equation which corrects two properties of real gas: the excluded volume of gas particles and attractive force between gas molecules. The van der Waals equation of state proposes the ideal gas law $ PV = nRT $ where the value of these constants approaches zero.

Complete step by step solution
The van der Waal equation is represented as: $ (P + \dfrac{a}{{n{V^2}}})(V - nb) = nRT $
Where $ P $ is the pressure, $ V $ is the volume, $ R $ is the universal gas constant, $ T $ is the absolute temperature and $ n $ is the total number of moles.
It was derived by van der Waal in 1873. It modifies the ideal gas law ( $ PV = nRT $ ). The constants $ a $ and $ b $ have positive values and are specific for each gas.
 $ ' $ $ a $ $ ' $ corrects for intermolecular attraction. Attractive force between molecules decreases the pressure of real gas, slowing the molecules and reducing collisions with the wall. Higher the value of $ a $ , greater will be the attraction among molecules and the more easily gas will compress.
 $ 'b' $ term represents the excluded volume of the gas or the volume occupied by the gas particles.
Hence the correct option will be (D): attractive force and volume of molecules.

Note
Notice that the van der Waal equation will become the ideal gas law when these two constants approach zero. Ideal gas law was derived by keeping these things in mind: $ 1) $ there is no intermolecular attraction between gas molecules. $ 2) $ the volume occupied by the gas is nearly zero. $ 3) $ the gas particles move randomly in agreement with newton $ ' $ s laws of motion. But real gases do not behave in the same way. They occupy some volume, have some interaction (attractive or repulsive forces) between molecules. So, the correction terms came to existence where $ 'a' $ accounts for attractive forces and $ 'b' $ accounts for volume of molecules.