2 ml of stock antibiotic (64 mg/ml) was diluted into a test tube containing 2 ml of broth (final volume is 4). A series of 2-fold SERIAL DILUTIONS were performed by transferring 2 ml to three other tubes, consecutively (final volumes now 2 ml in each tube). The final concentrations of antibiotic in the four tubes are: 32, 16, 8, and 4.

Question

2 ml of stock antibiotic (64 mg/ml) was diluted into a test tube containing 2 ml of broth (final volume is 4). A series of 2-fold SERIAL DILUTIONS were performed by transferring 2 ml to three other tubes,  consecutively (final volumes now 2 ml in each tube). The final concentrations of antibiotic in the four tubes are: 32, 16, 8, and 4.

Next, 2 ml of water were added to each each tube individually (not a serial dilution). What is the final concentration of antibiotics in the tubes?

MathMate · Answer

Make a table and everything would be clear.

stock totVol concentration
2 4 32 (2*64/4=32)
2 8 16 (2*64/8=16)
2 16 8
2 32 4

Now if 2 mL of water are added to each, then concentration would be:

stock totVol concentration
2 6 32 (2*64/6=21.3)
2 10 16 (2*64/10=12.8)
2 18 8 ...
2 34 4 ...

Complete the above table to get the concentrations (mg/mL)