[Bjornson]

Our lab balances are not accurate for amounts <10mg. So I've been told to make a stock of 25mg/100ml of the needed ingredient, then I should use the following:  25mg/100ml=2.5mg/10ml
10ml of that stock put into the one liter volumetric will give me the 2.5mg I need in my main stock solution.  I'm confused!  Can someone explain this better?

[nj_bartel]

Think about it visually.  If you take 100 molecules and put them in a liter of water, the molecules disperse so that they are evenly space throughout the liter.  When you pour 1/5 of that liter, you get 20 molecules.  You can then dilute those 20 molecules in 1/5 of a liter to a full liter, so that you have 20 molecules in 1 liter.

[AccordionGirl]

Right. If you want a solution that's got 2.5 milligrams for every 10 milliliters, but you must measure 10 times as much mass, you must increase the volume by 10 to keep the ratio the same.
Then, taking one tenth of that should get you back to 2.5 milligrams per 10 ml.

You can then add as much water as you want, but the mass of solute won't change. If you want to end up with 2.5 milligrams per liter, just make sure you have 2.5 milligrams, and then dilute to the needed volume.