1. (10pts) One of the solutions used to make bacterial cells competent is CaCl2

Question

1. (10pts) One of the solutions used to make bacterial cells competent is CaCl2 (molar mass = 110.98 g/mol). Your lab instructor will perform this task for you prior to the start of lab. However, you need to double-check your lab instructors’ calculations to ensure that the CaCl2 solution used to make your bacterial is the correct concentration. You lab instructor plans to make a 1M CaCl2 stock solution, that he or she will then dilute to a final working concentration of 350mM.

a. Your Lab Instructor has determined that 27.75g is required to make 250mL of 1M CaCl2. Is this correct? Show all of your work for full credit.

b. Your Lab Instructor has calculated that a 355mL of 1M CaCl2 is required to make a 350mM CaCl2 solution in 875mL total volume. Is this correct? Show all of your work for full credit.

Explanation / Answer

1. Volume = 250 mL = 0.25 L; Formula weight = 110.98 g/mol, Desired concentration = 1M

Mass = Concentration (mol/L) * Volume (L) * Formula Weight (g/mol)

Mass = 1 M * 0.25 * 110.98 = 27.745 g

Instructor is almost correct.

2. M1 = 1M, M2= 350 mM = 0.35 M, V2 = 875 mL

M1V1 = M2V2

1 * V1 = 0.35 * 875

V1 = 306.25 mL

Instructor is wrong in this case as she/he has estimated 355 mL instead of 306.25 mL

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