placidness vs placidity

placidness and placidity both are nouns.

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

As nouns, placidness and placidity are synonyms defined as:

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

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