That is a tough question to answer with the info you are providing. An ounce of Chinook boiled for an hour in a 1 gallon batch will certainly pack a strong IBU punch. If you boil 1.5 gallons and end up with 1 gallon left at the end, at the Chinook is 13% AA, you've got 100+ calculated IBU's (I come up with 106.1).
What style of beer are you brewing? Or, more relevant to the question you are asking, do you have a target number of IBU's you are trying to hit?
Brewing software can really help you out with a question like this. You can enter the hops you want to use, amount of water, length of boil, alpha acid content and it will give you the calculated IBU's. Then you can make adjustments as needed (reduce amounts, user later additions, split up the ounce into multiple additions...) to get closer to your target. There are some websites that do this - I've heard of Brew Toad although I haven't used it - and there are programs you can download like BeerSmith and Brewtarget.