What is the part of speech for words suffixed with "ity"?

More specifically than being just nouns, nouns that are derived from adjectives by various different suffixes (-ness, -(i)ty, -(t/s)ion, -hood, -(e)ry, etc.) are known as abstract nouns, in that they refer to an entity that is not concrete or, as you yourself said in your comment, are not “a person, place, or thing”.

There are also abstract nouns that are not derived. Many of them, in fact. It is a fairly generic word class. When creating nouns by nominalising a verb gerund—i.e., taking the gerund running and using it as a noun, as in the phrase in the running—they usually become abstract nouns. There are exceptions (a winning is a concrete noun, as in the plural, one’s winnings), or cases where the resulting noun can be either depending on meaning (like a cutting from a newspaper, which is a concrete noun, as opposed to the cutting of a birthday cake); but most are abstract.

As regards your bonus question, the only abstract noun derived from the adjective empirical is empiricalness. Empiricality, while regularly formed, quite easily understood, and occasionally used (about 50,000 hits on Google), has not been picked up by any dictionaries and would probably be considered non-standard by most.

In a word: noun. Practical is an adjective; practicality is a noun.

As for empirical, the noun form is empiricism.