The tough part was finding density at 80 Â°F instead of the standard 60 Â°F. This table
http://www.elyenergy.com/pdf/CO37.pdf gives 0.03257 ft^3/lb at 80 Â°F. So your 100 lbs occupies 3.257 ft^3 (ignoring the amount as vapor above the liquid).
The volume of a cylindrical tank is (pi/4)*(d^2)*h but your dimensions are in inches. Converting to feet,
(pi/4) * 1^2 * 54/12 = 3.53 ft^3.
So tank is pretty full. I don't know what the requirements are for expansion volume in the tank for warmer temperatures, but this seems "close". NOTE: The volume calculations assume these are inside dimensions. If they are outside dimensions a correction for wall thickness is required.