This is a liquid or other dispersable product? Start with 15 gallons of water only. Pull the sprayer at the speed you intend to use when you spray the chemical. You can do it on concrete, a parking lot, or grass. Figure out how many square feet you cover with the 15 gallons of water (width spray boom covers, in feet, tims distance traveled, in feet).
Say you spray 6' wide and travel 2,250 feet before running the tank dry. Then you know 15 gallons at your ground speed covers 13,500 square feet.
There are 43,560 square feet in an acre. Figure out what part of an acre one tank covers. In my example above, 13,500 square feet is 30% of one acre. Use that number times the label rate per acre and mix that much into your 15 gallon tank, following the directions for doing so that are listed on the product label. In my example it is .30 x 1.5#.
You need to know the size of the area you intend to spray too before you start (ideally). This will tell you in advance how much chemical you need to complete the job and how many times you should have to refill.