It's Plural because five hundred pounds, its more than one pound. If its one hundred pound, then it's gonna be singular. Two or more things are plural. One is singular.

Plural most of the time (the box weighs 500 pounds; James spent 500 pounds on a new bike). However, we also say "the hunter killed a 500 pound bear."

5 hundred pounds is correct as my teacher said in a simple way when you definite the something you count make it plural i hope you understand :)