If I add 75 mL of water to 125 mL of a 0.15 M #NaOH# solution, what will the molarity of the diluted solution be?
Molarity refers to the number of moles of a substance per litre of substance.
125/1000=0.125 (This is the proportion of 1 litre that 125 ml s is, seems like a redundant step but it helps you keep track of your numbers)
0.15x0.125=0.01875 (This is the number of moles of NaOH in your 125 ml sample)
The next step is adding the 75 ml of water to your 125 ml sample. This brings the total volume to 200 ml.
The concentration is now 0.01875 moles in 200 ml, simply multiply by 5 to scale to a litre to get your molarity