In a five-fold serial dilution of a 20 ug/mL solution, all tube dilutions are 1/5. What is the substance concentration in the third tube of this series?

In the third tube, the concentration is 0.16 ug/mL

Explanation:

In the first step, the solution is diluted by 5. Then, the concentration will be

20 ug/mL / 5 = 4 ug/mL

Then, in the second step this 4 ug / ml solution is diluted by a factor of five again:

4 ug / ml / 5 = 0.8 ug/mL

This solution is then diluted again by 5 and the concentration in the third tube will be then:

0.8 ug/mL / 5 = 0.16 ug/mL

Another way to calculate this is to divide the original concentration by the dilution factor (5 in this case) elevated to the number of dilutions. In this case:

Concentration in the third tube = 20 ug/mL / 5³ = 0.16 ug/mL