# Chemistry

posted by Brittnay

Starting with solid Tris base (FW 121.1) and 1 M HCl, describe how you would make 2 liters of 200 mM Tris-HCl buffer pH 7.5. pKa of Tris 8.14

1. DrBob222

Let's call TRIS base, TNH2.
pH = pKa + log (base)/(acid)
7.5 = 8.14 + log b/a
b/a = 0.23 approximately. You do it more accurately.
So base = 0.23*acid

You want 2000 mL x 0.200 M = 400 mmols.
Therefore you want 0.23*400 = 92 mmols base.

.........TNH2 + HCl ==> TNH3^+ + Cl^-
I.........x......0.........0
C........-400...-400......400
E........x-400 ....0......400

If x-400 = 92, then x must be 492.
So you take 492 mmols TNH2, add 400 mmols HCl, add H2O to the 2L mark, and you have it. You calculate grams TNH2 from this and mL HCl from this (depending upon how strong the HCl is).
I recommend you start with the grams and redo those calculations on paper to make sure the pH will be 7.5 when you finish.

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