Question

How much water to be added to 450 ml of a solution 0.5 M to obtain a concentration 0.30 M?

Answer 1

0.5 mols/ liter * 0.45 liters = 0.225 moles
.225 moles= 0.30 mols/liter * V liters
V = 0.75 liters final volume
0.75 - 0.45 = 0.30 liters added
which is 300 mL

Answer 2

since the amount of solute does not change,
0.30(450+x) = 0.50*450
x = 300