impediment vs difficultness

impediment and difficultness both are nouns.

Word	Noun	Adjective	Verb	Adverb
impediment	Yes	No	No	No
difficultness	Yes	No	No	No

As nouns, difficultness is a hypernym of impediment; that is, difficultness is a word with a broader meaning than impediment:

impediment: something immaterial that interferes with or delays action or progress
difficultness: the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result

Other hypernyms of impediment include difficulty.

impediment (noun)	difficultness (noun)
any structure that makes progress difficult	the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result
something immaterial that interferes with or delays action or progress