placidity vs tranquility

placidity and tranquility both are nouns.

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

As nouns, placidity and tranquility are synonyms defined as:

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

placidity (noun)	tranquility (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