Answer:
500ml or 0.5l
Step-by-step explanation:
The concentration is 0.40 M so there are 0.40 moles in 1 l of the solution.
0.40 moles -> 1 l
but we have to go to 0.20 moles, 0.40/2 = 0.20 moles but we have to divide the volume also by 2 that makes 0.5 liters.