Valery S. answered • 04/30/20
A UCLA graduate who is passionate about teaching and medicine
Hello there,
It's a typical serial dilution question. You basically want to use the formula Mi x Vi = Mf x Vf, where M and V are morality and volume respectively and the i and f mean initial and final.
We also need to convert 100 µL to mL, that is 100 µL = 0.1 mL.
For our first dilution, M1 = X (the stock concentration - it's supposed to be known by reading what the label on the bottle says), V1 = 0.1 mL, M2 = ? (we want to find it), and V2 = 9.9 mL + 0.1 mL = 10 mL.
Therefore the concentration of you solution after first dilution is M2 = (0.1X)/10 = 0.01X, that is it would be 1% of the initial stock concentration.
You then use the new concentration, 0.01X, as your new Mi and use the same formula to find the next concentration:
0.01X x 0.1 mL = Mf x 10 mL
Mf = 0.0001X or 0.01% of your stock concentration.
In the same manner, the last dilution will yield you a concentration of 0.000001X or 0.0001% of your stock solution, where X is once again your stock concentration.
Based on the passage, you cannot determine the stock concentration or final concentration - you need to provide more information about the experiment, that is perhaps there is a certain contraction per colony ratio that would be known beforehand or something along those lines. There should be one extra piece of information to find out the concentrations. Otherwise, you start with an unknown stock concentration and end up with an unknown concentration in tube C. It's a dead end unless, like I mentioned before, there is some relationship to the number of colonies grown on the petri dish, etc.
Valery
