You are performing an enzymatic reaction to digest DNA and nothing works for you. You need to find out why. You added the enzyme and the DNA to the reaction yourself and you are sure you did it right. It must be the digestion buffer. The working solution for the digestion buffer should contain 400 mM Tris, pH 8.0 and 125 mM MgCl2. First, you ask your colleague how he made the digestion buffer and he says: “I mixed
0.2 ml of 2M Tris, pH 8.0 stock solution
0.25 ml of 5M MgCl2 stock solution
and I completed the solution with 0.55 ml water to make a final volume of 1ml.”

a) Before checking your colleague’s calculation, what important question do you have to ask first for this specific solution? (not a generic question about all solutions)

b) You are satisfied with his answer to a) and you believe he made a dilution error. You use the equation that applies to dilutions to check his calculations. Aha! you find the problem. What is it and how did you find out?
Note: you can either check the math or you can do the calculations yourself.