1
Dilutions Series problem set
1. In a protein assay, the original sample was first diluted 1/5. Then, 5 mL
of the diluted sample was mixed with 20 mL of reactants to give 25 mL
total. 5 mL was removed from the 25 mL mixture. The 5 mL contained 3
mg of protein. What was the concentration of protein in the original
sample?
2. Suppose that you begin with a culture of bacteria that contains 1 X 109
cells / mL. SKETCH a dilution scheme so that your final tube has a
concentration of 200 cells/ 0.1 mL. (Assume that you can pipet NO LESS
than 10 L accurately!!!)
3. Suppose you have 20 L of an expensive enzyme that has 1000
Units/mL. You are going to use the enzyme in an experiment that requires
tubes with 0.01 Units/mL of enzyme and each tube will have 5 mL total
volume.

How much enzyme will each tube require?

Will you be able to accurately measure this amount of enzyme?
2

Show how you can dilute 10 L of the original 20 L of enzyme so
that you can use it in your experiment. Use a diagram and words to
demonstrate your strategy for preparing the enzyme.
4. You need to prepare serial dilutions of a DNA sample. Each dilution
will be made using the previous dilution. You will make up 10 mL of
each dilution (this 10 mL includes the volume you will remove to
prepare the next dilution!) Fill in the chart below:
Overall dilution
of original DNA
sample
1/10
1/50
1/1000
1/5000
1/30,000
Stock used
Original DNA
1/10
1/50
1/1000
1/5000
Vol of stock
needed
Vol of buffer
needed