# Biochem

posted by Ross

You have prepared a 400 mL of a .210 M acetate buffer solution with a pH of 4.44.
1. Determine the concentration of both the acetate and acetic acid in the solution.
2. If you made this solution using solid sodium acetate (MW = 136 g/mol) and liquid acetic acid (17.6 M, referred to as glacial acetic acid), what mass of sodium acetate is required and what volume of glacial acid is required?

I got part 1 but could not figure out part 2.

1. DrBob222

I could have used your numbers if you had given pKa and ratio base/acid you used. As it is you must check what I have; it may not agree with the constants you used.
pH = pKa + log (base)/(acid)
4.44 = 4.75 + log (base)/(acid)
I obtained b/a = 0.49 or
(base) = 0.49(acid)
Then a + b = 0.210
Solve those two equations to obtain
(acid) = 0.14M
(base) = 0.07M
The volume of the solution is 400 mL or 0.400 L.
mols acid = 0.14 x 0.400 = 0.056
mols base = 0.07 x 0.400 = 0.028

Then grams acid = mols x molar mass and use density to determine volume acetic acid.
grams base = mols base x molar mass sodium acetate.
You may need to work through the whole problem again using the values of your first calculation. Post your work if you get stuck or have questions.

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