Hi there! Both ways are fine, but sit in a chair in more common. I hope this helped! :)
Sit on a chair, sit in an armchair.
When you say, "I am going to go sit in a chair.", sit would be a verb because that is the action.
The sooner defrosted shrimp are cooked after being exposed to room temperatures, the better. Fresh shrimp should be OK for a few hours, since they will be immersed in boiling water after being...
Sit on the chair