Question

How much water must be added to 6.0 mL of a 0.05 M stock solution to dilute it to 0.020 M?

Answer 1

you are diluting it .05/.02=2.5 times. That means, you want one part stock solution, 1.5 parts water.

here one part is 6ml, so you add 9 ml water. Stir, label it as .02M

Now what if it had asked to make 5 ml solution? Still, diluting it 2.5 times, so 2.5 parts into 5 ml is 2 ml per part, so take 2ml of stock solution, and 3 ml of water.