peace of mind vs peacefulness

peace of mind and peacefulness both are nouns.

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

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

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

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