How many milliliters of the stock solution do you need to prepare the dilution? How many milliliters of water need to be added?

Question

You are in charge of preparing 25.00 mL of an amoxicillin solution with a concentration of 5.0 g/mL. The lab has a stock solution of ampicillin with a concentration of 80.0 g/mL.

Answer 1

V1C1 = V2C2

V1 = initial concentration = x ml

C1 = initial concentration = 80.0 g/ml

V2 = final volume = 25.00 ml

C2 = final concentration = 5.0 g/ml

(x ml)(80.0 g/ml) = (25.00 ml)(5.0 g/ml)

x = 1.56 ml of stock solution

25.00 ml - 1.56 ml = 23.44 ml of water

With the correct use of significant figures, you would have only 2 sig. figs. in your answer to volume of stock, so it would be 1.6 ml (5.0 g/ml has only 2 sig. figs.). When subtracting this volume from 25.00 mls, we would limit our answer to one decimal place and arrive at 23.4 mls of water to be added.