1. MES (2-(N-Morpholino)ethanesulfonic acid) is a common buffer used in biochemistry labs to stabilize proteins. The pKa for MES is 6.09.

Question

a. What is the pH of 1 liter of a 200 mM MES aqueous solution? Clue: you will need to solve a quadratic equation. (2.5 points)
b. How much solid NaOH (in grams) needs to be added to the MES solution to make a buffer at pH 6.5? (1.5 points)

Answer 1

If we call MES just HS, then
...........HS ==> H^+ + S^-
initial....0.2.....0....0
change.....-x......x.....x
equil......0.2-x...x.....x
Substitute the equilibrium values into the Ka expression and solve for x, then convert to pH.

For the second part,
you know pH = pKa + log(base/acid)
(base) = (OH) that must be added
(acid) = 0.2-(OH)
I would plug that into the HH equation, solve for base and convert that to grams. I shall be happy to critique your work if you post it.

Answer 2

Thanks Doc!

Answer 3

Part A...
Solving for x we will get 4.028x10^-4
Plugging it in into pH equation, we will get pH of 3.39

Part B
5.76g