Question

A typical concentration of ampicillin used for bacterial selection in liquid LB is 100 microliter/mL. we prepared a 100 mg/mL ampicillin stock solution for your use. When you prepare a 5 mL overnight culture, what volume of ampicillin will you add to the LB

Answer 1

The typical ampicillin concentration used for bacterial selection is 100 μg/mL. Our stock is 100mg/mL. 100 mg is nothing but 100 x 1000 i.e. 100000 μg or 10⁵ μg, as 1 mg = 1000 μg. To prepare for 1 mL of LB, we must add a thousandth of our stock to 1 mL of the diluent (in this case, LB). Thus, we add 1 μl of the stock to 999 μl of LB, to get 100 μg/mL. Now, since we have 5 mL of overnight culture, we must add 5 times of the stock that we add to 1 mL. Thus, we must add 5 μl of the stock to 4.995 mL of the overnight culture, to get 100 μg/mL.