J.R. S. answered 02/27/19
Ph.D. University Professor with 10+ years Tutoring Experience
STP = 1 atm pressure and 273K. Since temp didn’t change we have Boyles Law.
P1V1 = P2V2
(1atm)(3.00L) = (0.5 atm)(V2)
Solve for V2 to get the answer. Read about Boyles Law to learn and understand more.