Calculation?

Question

You are required to make up 200ml of cell growth media containing penicillin.
Concentration of supplied penicillin: 100mM
Final concentration required in media: 10 microM

What is the volume of penicillin needed?

Im a bit confused, do i need to use this formula?

Conc 1 x Vol 1 = Conc 2 x Vol 2

Answer 1

You need 200 uL of 100mM penicillin.

The way I do calculations like this is to figure out what fold dilution you need:

10 uM = 0.010 mM
100mM/0.010 mM = 10000-fold dilution
Then you just divide your total volume of solution by the dilution factor:
200 mL media/10000 fold dilution = 200 ul

Hope that helps.

Answer 2

The final solution is 10,000 times more dilute, and you need 200ml*10uM = 2 micromoles of penicillin = 100mM*V

Therefore, you need 0.02 ml of the original solution, mixed with 199.98ml of media, which corresponds to a 10,000:1 dilution.