placidity vs serenity

placidity and serenity both are nouns.

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

As nouns, placidity and serenity are synonyms defined as:

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

placidity (noun)	serenity (noun)
a disposition free from stress or emotion	a disposition free from stress or emotion
a feeling of calmness; a quiet and undisturbed feeling	the absence of mental stress or anxiety