I use a AquaClear which I had from my freshwater days.
Last year I brought a bucket of redsea pro salt from a member which was rock hard and it mix up cloudy. I use a quick filter also from my freshwater days for the aquaclear powerhead and it clear up within about 15-20 min.

http://www.drsfostersmith.com/produc...fm?pcatid=4638

I could be wrong, but I think when you use salt that has hardened, some of the sodium bicarbonate turns into calcium carbonate, and that makes for the cloudy water, it will no longer dissolve. Check the KH of your new salt water.
Hopefully some of the keener chemists can elaborate

I would also not use salt that's only mixed for 10 minutes. I mix mine a minimum of 6 hours, but preferably overnight..

I had no problem using the salt. Ca, and KH/Alk and PH were in there acceptable range. Fish, invert and coral all seem normal when I was using this salt.

I find that different brands of salt mix at different rates. I always let my salt run for a day but I also use fairly inexpensive salt. To your question, no 7.5 lbs of salt will not dissolve in 5g of water