I think you have to assume that the apple juice comes in at about 1.046 (the gravity of 1 lb of sugar dissolved in 1 gal. of water). I remember reading something about it here
. From that you can calculate how much the sugar you added would increase that.
(Potential x weight)/volume = gravity increase
So for corn sugar at 46 potential, that gives you:
(46 x 2lbs)/5gal = 18.4
Add that to your 1.046 and you wind up at around 1.064. If you assume that you will ferment down all the way to 1.000 (which it probably will, if not lower), that gives you about 8.4% alc.
This is all an estimation. If you've got any apple juice left over you could measure the gravity of it, or you could take a look at the bottle and try to figure out how much sugar is in there from the nutrition facts. Also, if you added the entire 5 gals of apple juice and the sugar, the sugar will displace some volume, and you'll actually wind up at more than 5 gal, which would drop your ABV, but not by a terrible lot.