A chemist wants to prepare a stock solution of H2SO4 so that a sample of 20.00mL will produce a solution with a concentration of .50 M when added to 100.0mL of water.

A)what should the molarity of the stock solution be?
B)If the chemist wants to prepare 5.00L of the stock solution from a concentrated H2S)4, which is the 18.0 M, what volume of concentrated acid should be used?

(please explain how you get the answer..I'm so confused, help!!)

1 answer

A. The easy way is to use the dilution formula. Also you MUST assume that the volumes are additive.
mL1 x M1 = mL2 x M2
0.5 x 120 = 20 x M2
M2 = 0.5 x 120/20 = 3M

B. Use the dilution formula again.
18M x mL1 = 5,000 mL x 3M.
Solve for mL1 which is volume of th 18 M stuff.