apropos vs appropriateness

apropos is not a noun while appropriateness is a noun.

Word	Noun	Adjective	Verb	Adverb
apropos	No	Yes	No	Yes
appropriateness	Yes	No	No	No

apropos (noun)	appropriateness (noun)
	the quality of being specially suitable
	appropriate conduct; doing the right thing

apropos (adjective)	appropriateness (adjective)
of an appropriate or pertinent nature

apropos (adverb)	appropriateness (adverb)
at an opportune time
introducing a different topic