Question

Suppose you want to test the results of a transformation by growing Escherichia coli cells in LB medium containing ampicillin as the antibiotic for selection. Ampicillin at a concentration of 100 µg/mL will kill cells that do not contain an ampicillin resistance gene, but will allow the growth of cells that have been transformed with this gene. The concentrated stock of ampicillin is 100 mg/mL. How many microliters of the ampicillin stock should you add to 50 mL of LB for a bacterial culture?

Answer 1

Answer:

50.0 μL

Explanation:

When a dilution is done, the mass of the solute (in this case the ampicillin) remains constant, following the Lavoiser's law that the mass is conserved. The mass is the concentration (C) multiplied by the volume (V), so if 1 is the stock solution, and 2 is the bacterial culture after the addition of the antibiotic:

m1 = m2

C1*V1 = C2*V2

C1 = 100 mg/mL = 100000 μg/mL (1 mg = 1,000μg)

C2 = 100 μg/mL

V2 = 50 mL + V1 = 50000μL + V1 (V1 in μL)

100000*V1 = 100*(50000 + V1)

1000V1 = 50000 + V1

999V1 = 50000

V1 = 50.0 μL