Question

you need to prepare 5ml NaCl solution . you already have a stock solution of 10% NaCl that you are asked to use . how many ul NaCl solution would you need to make 5ml of a 2% NaCl solution?

Answer 1

You want 1/5 the concentration, so you need 5 times the volume.

So, use 4 parts water and 1 part 10% NaCl.
That is, use 1mL NaCl and 4mL H2O