How to make a 0.00125 solution of NaOH using pipets and water?

Question

Using only 5.00, 10.00, and/or 25.00mL volumetric pipets, up to four 100mL flasks, and water, how would you make 100mL of a 0.00125M solutioin of NaOH from 1.0L of a 0.10M NaOH solution?

Answer 1

Because of the limitations from the apparatus, you can do this by diluting the NaOH twice: 1) dilute 0.1M of NaOH to 0.005M. 2) dilute 0.005M to 0.00125M

1. Put 5mL of 0.1M NaOH into a 100mL flask. Then fill the flask to 100mL by adding water.

0.1M * 5mL = 0.005M * 100mL

2. Put 25mL of the diluted NaOH (0.005M) into another 100mL flask. Then fill the flask to 100mL by adding water.

0.005M * 25mL = 0.00125M * 100mL

Answer 2

100 mL x 1.25x10-3 M = 1.25x10-4 moles NaOH This is the MOLES of NaOH you need in solution.
Each 5 mL (the smallest pipet) of stock solution provides .005x.1 = 5x10-4 moles of NaOH. So you probably want to do a 1:20 dilution. Take 5 mL of stock and add 95 mL of water into a 100 mL flask. Mark as 1/20 Stock. Now this has
2.5x-5 M NaOH. in a 5 mL pipet. You can pipet 25 mL of this stock and 75 mL water into a 100 mL flask for your 1.25x10-3 M solution.