For yarn that's to be knit on size 9s, a 5 is awfully small, will give you a pretty dense knit, and will take you a lot more stitches. It may also be hard on your hands. But you will need to make a sample and figure out how many sts per inch you get and multiply that times 80 to see how many stitches you need. I might suggest you use a size 8 needle, that's much more appropriate for the yarn; a too small needle makes it harder to work the stitches.
sue- knitting heretic