If 5 GB or RAM is used out of 8 GB and then if i want to run a application which is 32 bit compatible then can it run on this computer in this scenario?
Short answer: Yes you can
Long answer: Applications (as opposed to the OS kernel components) do not see the "real" (or physical) memory layout. The Computer's MMU is instructed by the OS to create a unique "fake" (or virtual) memory layout for the application, that is essentially empty. The process will simply not see the RAM that is already used (apart from Statistics)