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Quiz 6 Answer Key
(Chapter 13: Dilutions)
1. How would you prepare 25 mL of a 1/15 dilution of food coloring?
1
=X
X = 1.7 mL
15
25 mL
Take 1.7 mL of food coloring and BTV 25 mL
2. If you take 5 mL of 250 mM NaCl and add 25 mL of water to it, what is
the final concentration of NaCl in the dilution?
(5 mL)(250 mM) = (30 mL)(y)
y = 41.7 mM NaCl
3. If you prepare a 1/40 dilution of a 350 mg/mL solution, what is the final
concentration of the solution?
350 mg/mL X 1/40 = 8.75 mg/mL
4. You do an assay (test) to determine how much protein is present in a
sample. To perform the assay, it is necessary to dilute the original sample
1/100. If the assay shows that the concentration of protein in the diluted
sample was 1.5 mg/mL, what was the concentration of protein in the
undiluted sample? ____
1.5 mg/mL X 100 = 150 mg/mL
5. A flask contains 500 mL of solution. A small amount of sample was
removed from the flask. This sample was diluted 1/50. The concentration
of protein in the diluted sample was 2.5 mg/mL.
a. What was the concentration of protein in the original sample? Express
your answer in units of mg/mL.
2.5 mg/mL X 50 = 125 mg/mL
b. How much protein was present in the entire 500 mL ?
125 mg/mL X 500 mL = 62,500 mg
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6. You take 100 mL of suspended bacteria and grind them into a cell paste.
Then, you remove 100 L of these cells and perform a test that tells you how
much protein is present. Your test shows that 30 g of protein are present in
the 100 L of cell paste. What was the concentration of protein in the cell
paste?
a. Express your answer in units of g/L.
0.3g/L.
b. Express your answer in units of mg/mL.
0.3 mg / mL
7. Suppose you perform the following dilution series:
Step 1: You take 100 L of a bacterial culture and add 0.9 mL of
media.
Step 2: You take 100 L of the dilution prepared in step 1 and add
9.9 mL of media.
Now, suppose you plate 0.1 mL of the dilution made in step 2 onto a
plate and observe 555 colonies.
A. Provide a sketch of the steps listed above (including the plating
step.)
Step 1 is a 1/10 dilution.
Step 2 is a 1/100 dilution.
B. What is the concentration of bacteria in the ORIGINAL
(undiluted) culture?
Therefore, the final dilution is 1/10 X 1/100 = 1/1000
Concentration = 555 bacteria/ 0.1 mL X 1000 = 5.55 X 106 bacteria /
mL