Question

Why is for ideal gas the potential energy associated with its molecules zero?

Answer 1

When scientists started studying gases and it's behavior, it became quite difficult for them as there are so many factors associated with the behavior of real gas, that makes it hard to study.

So to simplify things , they imagined a gas which has no interaction between it's molecules along with many other ideal properties.

Potential energy is only defined for a system in which internal force do some work, and here is no force at all… Hence its potential energy is zero.