You ask your lab assistant to prepare 1L of solution Zeta at a concentration of

Question

You ask your lab assistant to prepare 1L of solution Zeta at a concentration of 1 M. He comes up to you with a 0.5 M stock solution and asks for help in calculating how much to use. What do you tell him?

2.   Illustrate, as on your first lab calculation handout, how you would make 3 serial 4-fold dilutions of an original stock solution. Indicate for each tube (you can set up a table beneath the diagram), including the original stock solution, the following information:

A. What is the n-fold dilution for each tube ?

B. What is the relative concentration for each tube ?

C. What is the actual concentration in each tube if the original concentration = 1 M ?

D. If the original tube has 400 cells per mL, how many cells per mL are in each dilution tube ?

E. If you pipetted 0.25 mL of the second dilution tube into a cuvette, how many cells would there be in the cuvette ?

Explanation / Answer

Answer 1 The required solution i.e. 1 M could not be made by using 0.5 M as stock because stock has to be of higher concentration than the required solution.

If stock would have been 5 M then we could have diluted it to make 1M by the formula

Ms x Vs = Mrx Vr

where Ms and Vs are molarity and volume required of a Stock solution while Mr and Vr are molarity and volume of required solution

So, 5 x Vs = 1 × 1

Vs = 1/5 = 0.2 liter i.e. 200 ml

for making 1 liter of 1M solution we should take 200 ml of 5M solution and add 800 ml of water to it.

Answer 2A

3 serial 4 fold dilution means diluting a solution to 1/4 of its concentration back to back 3 times

i.e.first dilution is 4 times of stock

second is 4 times of first or 16 times of stock

third dilution is 4 times of second or 64 times of stock.

So, n value is 4, 16 and 64 with reference to stock

Answer 2B

Relative concentration in tube 1 (first dilution) is 1/4 = 0.25 relative to stock

Relative concentration in tube 2 (second dilution) is (1/4)/4 = 1/16 = 0.625 times relative to stock

Relative concentration in tube 3 (third dilution) is (1/16)/4 = 1/64 = 0.015625 times relative to stock

Answer 2C

If stock is 1M

tube 1is 0.25 relative to stock so actual concentration 0.25 M

tube 2 is 0.0625 relative to stock so actual concentration is 0.0625 M

tube 3 is 0.015625 relative to stock so actual concentration is 0.015625M

Answer 2D

If stock has 400 cells per ml

tube 1 is 1/4 of stock so it has 400/4 =100 cells per ml

tube 2 is 1/16 of stock so it has 400/16 = 25 cells per ml

tube 3 is 1/64 of stock so it has 400/64 = 6.25 or 6 cells per ml

Answer 2E

Tube 2 has 25 cells per ml

If 0.25 ml is taken, number of cells would be

1 ml has 25 cells

0.25 ml has (25 /1) × 0.25 = 6.25 or 6 cells

So, 6 cells will be there in cuvette.

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