"Empty" or "Vacant" or "Free"

With the word ‘chair’, you can use either 'empty', 'vacant' or 'free'.

You could also say 'is this chair taken?'

In AmE, 'free' is used more frequently than 'vacant'.

'Empty' is used mainly for containers (bottles etc) but can also mean "without people"

For homes, flats, hotel rooms etc., use 'vacant'.

'Free' is ambiguous. It also means 'free of cost'.

For example: -The house is vacant -> It is for sale.

-The house is free -> You can have the house free of cost.

1. The chair is free.

2. The chair is empty.

3. The chair is vacant.

4. The chair is unoccupied.

All of the above sentences mean the same... (1. can be ambiguous).

If you see that nobody is actually sitting on that chair, but you are not sure whether it has been free to use because somebody may have occupied it before and have just gone out and may come back to use/occupy it again.

For the above situation, use 'vacant' or 'unoccupied'.