Recently, I’ve seen sentences along the lines of “this will reassure you and give you piece of mind”, so I think it’s time for us to have a look at this troublesome pair: peace and piece. They come with two related phrases. Well, phrases that are related to each word, but actually mean very different things: “peace of mind” and “a piece of [someone's] mind” and this is possibly how the two have come to get mixed up.
Peace means tranquillity, freedom from disturbance, and, most importantly, freedom from or the cessation of war or other conflict. “After the war, came peace”. “Peace of mind” means reassurance, tranquillity, knowledge that all is well. So good house and contents insurance or saving up some money to live on if you lose your job is likely to give you peace of mind.
A piece is a little bit of something. A piece of cake; a piece of the action. And to give someone a piece of your mind is to rebuke them or tell them off. A bit different from the peace of mind in the first example!
So, if I give you a piece of my mind (grrr) then you might well be lacking in peace of mind for a while!