posted by Helen .
What volume of a 0.5 M solution is required to prepare 250 mL of a 0.15M solution?
(Note: M = mole per Litre)
You want to dilute it by a factor of .5/.15, or 3.33 times. So you need one part original, 2.33 parts water to dilute.
What is one part? 250/3.33=75ml
so you need 75 ml of the original solution. Add 75*2.33=175ml water, stir, and label.