The value of van der Waals constant a for a gas is zero? What does it signify?

The unit of van der waal’s constant ‘a’ is :

What are van der Waals' forces?

Find out the value of the van der Waals constant 'a' when two moles of a gas confined in a 4 L flask exerts a pressure of 11.0 atm at 300 K. (b= 0.05 L mol^(-1)) .

Assertion: The value of vander waal's constant a is higher for NH_(3) than for N_(2) Reason : Intermolecular H-bonding is present in NH_(3)

How are the van der Waal's constant 'b' related to the molecular size?