peacefulness vs peace of mind

peacefulness and peace of mind both are nouns.

Word	Noun	Adjective	Verb	Adverb
peacefulness	Yes	No	No	No
peace of mind	Yes	No	No	No

As nouns, peacefulness and peace of mind are synonyms defined as:

Other synonyms of peacefulness include ataraxis, heartsease, peace, repose, serenity.

peacefulness (noun)	peace of mind (noun)
the absence of mental stress or anxiety	the absence of mental stress or anxiety
a state that is calm and tranquil