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Dilutions Series problem set answer key
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?
Overall dilution is: 1/5 * 1/5 = 1/25
Final concentration of dilution is 3 mg/5 mL
Therefore, original concentration = 3 mg/5 mL (25) = 15 mg/mL
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!!!)
What dilution needs to be made?
109 / 2 X 103 = 500,000
We need to make a 1/500,000 dilution.
An example is:
10 L cells + 990 L media (Dilution 1: 1/100)
10 L Dilution 1 + 990 L media (Dilution 2: 1/100)
20 L Dilution 2 + 980 L media (Dilution 3: 1/50)
Overall dilution is now 1/100 * 1/100 * 1/50 = 1/500,000
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?
Each tube need 5 mL (0.01 units/mL) = 0.05 units
2

Will you be able to accurately measure this amount of enzyme?
0.05 units (1 mL/1000 units) = 0.00005 mL.
This is equal to 0.05 L. This cannot be pipetted accurately!!

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.
If we dilute the original enzyme by 1/100, we will need to use 100X the
original volume, or 5 uL. This can be pipetted accurately. Therefore,
Take 10 uL of original enzyme and add 990 uL buffer. This is a 1/100
dilution.
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
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
1 mL
2 mL
0.5 mL
2 mL
1.67 mL
Vol of buffer
9 mL
8 mL
9.5 mL
8 mL
8.33 mL