Question

A chemist wants to prepare a stock solution of H2SO4 so the samples of 20ml will produce a solution with concentration of 0.5M when added to 100ml of water. What should the molarity if the stock solution be?

Answer 1

If you want the first option you need 2.5M.

If you want the second, you will end up with 120ml so your starting solution is:

0.5x (120/20) = 3.0M

This assumes that there’s no loss of volume on mixing. To remove that uncertainty, start with the 20ml of acid and add water, mixing, till you get the right volume.