differences between different noun forms of same adjective

Solution 1:

Suffixes -ness and -ity are roughly synonymous, but they have different etymologies.

There are two roughly synonymous suffixes, -ness and -ity, which are typically used for forming abstract nouns from adjectives, as in example below.

• generous + -ness = generousness

• generous + -ity = generosity

The first suffix, -ness, is etymologically native, while -ity entered the language as a result of contact with French during the Middle English period.

(..)

In addition, the meaning of words in -ity is often not entirely compositional, i.e., not deductible from the meanings of the base and the suffix. Thus, it is both phonologically and semantically more opaque than -ness (cf. Riddle 1985: 443–444; Aronoff and Anshen 1998: 246).

This was extracted from the article PRODUCTIVITY OF THE SUFFIXES -NESS AND -ITY IN 17TH-CENTURY ENGLISH LETTERS: A SOCIOLINGUISTIC APPROACH.