Question

In the Vander Waals equation, ’a’ signifies:
A. Intermolecular attraction
B. Intramolecular attraction
C. Attraction between molecules and wall of container
D. Volume of molecules

Answer 1

Ideal gases are the gas which follows the ideal gas equation i.e. $PV = nRT$ which represents the actual pressure-volume-temperature relationship of gases while in the real gas equation some deviations take place in the equation because the molecules in the gases interact with each other. Thus, the equation formed is called the Vander Waals equation.

Complete answer:
Gases is one of the forms of matter which consist of a large number of identical particles (atoms or molecules) that are so small and so far apart on the average that the actual volume of the molecules is negligible in comparison to the empty space present between them. Therefore, they are considered as point masses. But in real gas, there are some interactions that take place between the molecules due to which some deviation occurs in the Ideal gas equation. Thus, the equation formed is:
$\left( {p + \dfrac{{a \, {n^2}}}{{{V^2}}}} \right)\left( {V - nb} \right) = nRT$
where p is pressure; n is number of moles of gas; V is volume; T is temperature, a and b are the constants and their value depends on the characteristic of a gas.
Value of a in this equation is the measure of magnitude of intermolecular attractive forces within the gas and is independent of temperature and pressure.

**The correct option is A i.e. Intermolecular attraction.

Additional information: **
1. At high pressure, molecules do not strike the walls of the container with full impact because of which the pressure exerted by the molecules on the walls of the container is affected. Thus, the pressure exerted by the real gas is lower than the pressure exerted by the ideal gas.
2. At very low temperature, intermolecular forces become significant. As the molecules travel with low speed thus can be captured by one another due to attractive forces.

Note:
real gases show ideal behavior of gases when conditions of temperature and pressure are such that the intermolecular forces between the molecules are practically negligible. The real gases show ideal behavior when the pressure approaches zero.