Possessive Pronouns

Good sentences! Just a few corrections:

Is this her place? I'm sorry, I'll take the other chair.
This is his cellphone.

This is its cat. ("its cat" suggests that the owner of the cat is not human. This is possible if for example, we're talking about a robot who owns a cat. But if you mean to say the cat is owned by a person, you must use my/your/his/her/our/their)
This is its dog. (the same applies for this sentence. If the dog is owned by a person, use my/your/his/her/our/their)

These are your exercises about tenses, which you have to do for the homework (i don't know if it is the usually way to say it in English)These are their books. Are these their books? Yes, their they are.
 
This lip is his (This isn't wrong, but just to clarify, when you say "lip", you are referring only to either the upper lip or lower lip, not both.)
This These t-shorts t-shirts are ours
This These boots are theirs
This These overcoats are yours (The singular form is "This overcoat is yours")

Who is this car Whose car is this? The car is mine.
These are her dresses 

The wheel of the bike. (This isn't wrong but we usually just say "the bicycle wheel")
The string of the guitar (This isn't wrong either, but we usually just say "the guitar string/strings") (I love to play the guitar) 
The tom tom-tom (drum) of the drum set (Yey!)