0.4M * 500mL = 0.1M * V2
Solving for V2:
V2 = 2000mL
Since we started with 500mL, we need to add:
2000mL - 500mL = 1500mL
So, we need to add $\boxed{1500\text{mL}}$ of water.
2) For the NaOH solution, we can use the same formula:
M1V1 = M2V2
6.0M * 120mL
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