Hi there. I'm sorry about the loss of your betta. 7 Mollies in 5gal is really overstocked., so if your divider had perforations then your betta was also exposed to their wastes and may not have been able to tolerate the water quality. Since your betta has passed, I would remove the divider so at least the Mollies can have the full 10gal. Even then, that's still a crowded tank.
You can purchase water testing kits online
from any aquarium store's site. This is the kit you will need: http://www.petguys.com/-317163010340.html
For a helpful tank stocking calculator, take a look at this site: http://www.aqadvisor.com/AqAdvisor.php