It depends who you are around. I know people who curse casually and people who never curse. If you are with a group of people who curse a lot "Put your ass on that chair" would be informal. Usually if you said "Put your ass in that chair" people would think that you are angry at them. A more average way of saying "Please, sit down on that chair" would be just saying "sit." or asy Kaedee said "You can sit there."

Haha you aren't wrong, but if you want to know the phrases that are more common in the US I added them below. Formal: Please have a seat. Informal: You can sit there. Rude/very informal: Sit your ass down.

Haha :)

OMG, that sounds very rude to me!