posted by meghan .
. Use the Henderson-Hasselbach equation:
pH = pKa + log [A-]/[AH], where A- is the conjugate base and AH is the weak acid.
Calculate the pH of an acetate buffer in which the acetate concentration [CH3COO-] is 3.5 times greater than acetic acid [CH3COOH]. pKa = 4.74.
9. How would you prepare a 0.6 M acetate buffer with a pH = 4.0?
pH = pKa + log(acetate)/(acetic acid)
You want (acetate)*3.5 = (acetic acid) so I would let CH3COOH = x and CH3COO^- = 3x
Substitute into the HH equation and solve for pH.
Use the HH equation, substitute Pka and pH and solve for base/acid ratio.
The second equation you need is
acid + base = 0.6M
Solve those simultaneously to obtain base and acid, then use those values to prepare the solution from CH3COOH and CH3COONa (Note: You can use CH3COOH and NaOH but that requires more math.) Do it the easy way by weighing out CH3COOH and CH3COONa.