The possessive S should not be before the noun that it belongs to?

I'm solving English grammar exam (MCQ) and one of the questions there is about the next sentence:

This car is my car, and that car is Tom's.

According to what I know the possessive s should come before the name rather than after him or in the end of sentence. Therefore it should be Tom's car rather than car Tom's. Then I'm not sure if what I see in the exam is just a mistake or it has explanation.