Find active mass of 4gm of H2.

Question

Please solve it and explain it.

Answer 1

There has to be some chemical reaction associated with this

Answer 2

In book this line is written in the theory and answer is 2 moles /dm^3 given.
But I don't know method of finding.
So please tell me how I solve it.

Answer 3

I've never heard of "active mass" until now. When I looked it up on Google I found some information to the effect that is was the molarity. So 4 g H2 is 2 mols and if that is in 1 L (1 dm^3) that will be 2 M.