placidity vs tranquillity

placidity and tranquillity both are nouns.

Word	Noun	Adjective	Verb	Adverb
placidity	Yes	No	No	No
tranquillity	Yes	No	No	No

As nouns, placidity and tranquillity are synonyms defined as:

Other synonyms of placidity include quiet, repose, serenity, tranquility.

placidity (noun)	tranquillity (noun)
a disposition free from stress or emotion	a disposition free from stress or emotion
a feeling of calmness; a quiet and undisturbed feeling	a state of peace and quiet
	an untroubled state; free from disturbances