placidity vs placidness

placidity and placidness both are nouns.

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

As nouns, placidity and placidness are synonyms defined as:

placidity and placidness: a feeling of calmness; a quiet and undisturbed feeling

placidity (noun)	placidness (noun)
a disposition free from stress or emotion	a feeling of calmness; a quiet and undisturbed feeling
a feeling of calmness; a quiet and undisturbed feeling