How do you calculate the dilution factor in a serial dilution series and apply it to determine the final concentration?

• A. Multiply the dilution factors of each step; e.g., 1:10 at each step yields a cumulative dilution of 10^n; final concentration = initial concentration divided by cumulative dilution
• B. Add the dilution factors
• C. Multiply the final concentration by the cumulative dilution
• D. Subtract from the initial concentration

Answer: (A)

In a serial dilution, each step reduces the concentration by a specific dilution factor, and the overall reduction is found by multiplying those factors together. After n steps, the cumulative dilution is the product of each step’s dilution factor. If every step is a 1:10 dilution, the cumulative dilution becomes 10^n. The final concentration is the initial concentration divided by that cumulative dilution. For example, after 3 steps of 1:10, the final concentration is initial concentration divided by 1000. If you start with 100 μg/mL, you end up with 0.1 μg/mL. This reflects how dilutions compound: each step scales the concentration by the step’s factor, so you multiply the factors to get the total dilution and divide the starting concentration by that total. Adding factors, or multiplying the final concentration by the cumulative dilution, or subtracting from the initial concentration, do not describe how serial dilutions affect concentration.