The question is not correct. To dilute a stock solution the stock solution must be more concentrated. You cannot dilute a 2mM stock solution to get 25mM or 5 mM.
Here is something you can do to explain serial dilution:
Add 200 ml of water each into beakers A, B, C and D.
Add 200 mL of 2 mM stock solution into beaker A. Mix. Beaker A now has 400 ml of 1mM methyl red. Now take 200 ml from beaker A and add to beaker B. Mix. Beaker B now has 400 ml of 0.5 mM. Take 200 ml from beaker B and add to beaker C. Mix. Now beaker C has 400 ml of 0.25 mM. Take 200 ml from beaker C and add to beaker D. Mix and now beaker D has 400 ml of 0.125 mM. You can continue to do this. In each case the dilution factor is 2 (200 ml methyl red added to 200 ml water i.e., 200 ml diluted to total volume of 200+200 = 400 ml giving dilution factor of 2). Dilution factors of 2 are commonly used in serial dilutions to make lower concentrations from a high concentration stock solution. Hope this explains the concept of this important skill of serial dilution that is often used in biology/biochemistry experiments.
