(1) Here are the steps you take to do a serial dilution starting with a 100%(v/v) substance in solution.
1. You add 1 mL of this starting solution in tube #1, then Q.S. it to 100 mL.
2. Next you do serial 4-fold dilutions in three more tubes (tubes #2, #3, #4), and the total volume in tube #4 when you finish is 30 mL.

What volume of diluent is in tube #4?

(2) You have a substance in solution that has been serially diluted 1/8 in three cuvette tubes. Using the standard curve you built for this substance, you determine that the third tube contains 0.068 mg/mL of this substance. What was the concentration of the undiluted "starting" substance?

(3) You need to make a serial 1/2 dilution in three tubes, and the final volume of the third tube needs to be 20 mL. What volume will you take from the second tube to put in the third tube with diluent?

(4) You have a 0.26%(w/v) substance in solution. What is the concentration of this solution in micrograms per microliter?