Answer:
500 mL water
Explanation:
M1V1 =M2V2
M -molarity, V - volume of solution before adding water
M1 = 12MV1 = 250 mL
M2 = 4.0 M
12M * 250 mL = 4.0*V2
V2 = 12*250/4.0 = 750 mL of solution should become after adding water
750 mL - 250 mL = 500 mL of water should be added
