The second method (called the \"hard\" method by some) is to mix the acid and ba
ID: 1076676 • Letter: T
Question
The second method (called the "hard" method by some) is to mix the acid and base forms of a weak acid in solution to produce the exact pH needed. This has the advantage of producing a solution that is free from additional ions that might be brought in by the acid or base used to adjust the pH in the first method.
Calculate the grams of the acid and base forms of Tris needed to produce 200 mL of a 25 mM solution of buffered Tris at pH 8.0. The pKa for Tris is 8.2 and the molecular weight of the base form is 157.6, and the acid form is 121.14
Explanation / Answer
The pKa of Tris is 8.2 (Literature Value)
Using Henderson Hasselbach equation for acids,
pH = pKa + log [A-]/[HA]
8.0 = 8.2 + log [A-]/[HA]
-0.2 = log [A-]/[HA]
0.63 = [A-]/[HA]
This means
[A-]/[HA] = 0.63/1.0
% A = 0.63/(1.0 + 0.63) = 38.6%
Total Tris = 5 mMol (200 * 25 mM)
therefore, mMol of Acid = 38.6% of 5000 mMol = 1930 m Mol = weight of acid = 1930 mMol * 121.14 = 233.8 grams Acid
mMol of base = 5-1.93 = 3070 mMol Base = weight of base = 3070 mMol * 157.6 g/mol = 483.83 gram base