immobility vs inertness

immobility and inertness both are nouns.

Word	Noun	Adjective	Verb	Adverb
immobility	Yes	No	No	No
inertness	Yes	No	No	No

As nouns, inertness is a hyponym of immobility; that is, inertness is a word with a more specific, narrower meaning than immobility:

Other hyponyms of immobility include immotility, immovability, immovableness.

immobility (noun)	inertness (noun)
the quality of not moving	immobility by virtue of being inert
remaining in place