Question

You need to make 1L of LB plates with 50ug/ml ampicillin. If your ampicillin stock is 100mg/ml how much ampicillin do you need to add to the media?

Do you think adding this amount of antibiotic requires you to adjust the volume of the media (i.e. will the media be diluted significantly)?

Answer 1

we know that

mass of ampicillin = concentration x volume

given

volume = 1 L = 1000 ml

so

mass of ampicilin = 50 x 10-6 g x 1000

mass of ampicilin = 50 x 10-3 g

mass of ampicilin = 50 mg

so

we need 50 mg of ampicilin from the stock solution

now

given that stock solution is 100 mg / ml

so

volume of stock solution required = mass of ampicilin / conc of stock solution

so

volume of stock solution required = 50 / 100

volume of stock solution required = 0.5 ml

so

0.5 ml of ampicilin stock solution is added

no adjustment of volume is required , because

when 0.5 ml is added to 1000 ml of media

the resulting volume is 1000.5 ml which is negligible change