In the described serial dilution, what is the total volume after the first dilution step?

• A. 5 mL
• B. 20 mL
• C. 10 mL
• D. 1 mL

Answer: (C)

In a dilution step, you mix a portion of the sample with diluent to reach a new, larger volume—the total volume is simply the sum of what you added. If you transfer 1 mL of the sample into 9 mL of diluent, you end up with 10 mL of diluted mixture. That shows a 1:10 dilution, and the total volume after the first step is 10 mL. The other options would correspond to different amounts of diluent added (for example, 1 mL into 4 mL would give 5 mL total, and 1 mL into 19 mL would give 20 mL total).