Spelling of "possibility"
Solution 1:
The reason is this: the word possible has three syllables. In the transcription /pɒsɪbl̩/, we can see a little mark under the /l/ which indicates that that the /l/ constitutes a whole syllable on its own. There is an alternative, slightly child-like pronunciation for this word, /pɒsɪbəl/. This uses the schwa vowel before the /l/ to make the nucleus of the final syllable (we can think of the nucleus like the body of the syllable).
Syllabic /l/, like syllabic /n/ is only found in unstressed syllables like this. There is nearly always an alternative pronunciation available with a schwa vowel, /ə/ instead. Schwa, like syllabic /n/ and syllabic /l/, also only occurs in unstressed syllables.
Now, here's the point. The suffix -ity, which is used to change adjectives into nouns, involves a stress rule. It dictates that the stress in the word occurs on the previous syllable. In other words the stress must move to the last syllable of the root word. This kind of stress rule is very common with suffixes in English. For example, the suffix -tion dictates that the stress moves to the previous syllable in exactly the same way. We can see the stress move when we add the suffixes to the following examples (stress is indicated by bold font):
- con'taminate.
- contami'nation.
- ab'normal.
- abnor'mality.
This rule causes an issue with certain adjectives, however. The following words are usually said with a syllabic /l/ at the end:
- accessible
- reliable
- deniable
- responsible.
Now if we want to stick the suffix -ity onto the ends of these words to make nouns, our rule says that the final syllable of the original adjective must now be stressed. However, we can't do this because we cannot have a stressed syllable in English that is a syllabic consonant. If you try to say:
- possibleity
You will immediately notice that this sounds very odd - because it breaks the phonological rules of English. One solution to this is to put a vowel in the final syllable of the root. We can't however use a schwa, because as we noted above, schwa cannot occur in stressed syllables in English either. The solution to this problem is to insert the KIT vowel /ɪ/ in the final syllable. This allows us to put the stress on the correct syllable:
- accessi'bility
- relia'bility
- denia'bility
- responsi'bility
This is the sound being represented by the letter 'I'. Now that we have a stressable syllable here, everything is now ok with the world - sorry, I meant word.
Hope this is helpful!
Solution 2:
In some respect your question is justified as the Latin adjective is pos'sibilis/possibile, the French form is aready shortened pos'sible and English has the French form but the stress on the first syllable 'possible. The Latin noun possi'bilitas/ possibili'tat-is (genitive) gives possibili'té in French and possi'bility in English.
Link to etymonline possible and possibility
http://www.etymonline.com/index.php?allowed_in_frame=0&search=possible&searchmode=none
http://www.etymonline.com/index.php?allowed_in_frame=0&search=possibility&searchmode=none