The caffeine content of coffee varies greatly, depending on the beans, how they're roasted, and other factors, but the average for an 8-ounce cup is about 100 milligrams (mg).
No, 'cat' is a common noun, a word for any cat of any kind. A proper noun is the name of a person, a place, a thing, or a title; for example: Cat Deeley, host of TV series 'So You Think You Can Dance'.
Do not use a capital letter for a common noun, unless it starts a sentence. A common noun is the word we use for something, e.g., dog, ship, auntie. Common nouns contrast with proper nouns, which are specific names or titles, e.g., Rover, Titanic, Auntie Sally.