Question

Acetic acid has a pKa of 4.74. A stock solution of 1.0 M acetic acid is available. A stock solution of 2.0 M potassium acetate is available. Show all the calculations and explain how to make 560.0 mL of 250. mM acetate buffer that has a pH = 5.0. (Hint: first make a 250 mM potassium acetate solution and a 250 mM acetic acid solution. Then determine how much of each to use.).

pH = pKa + log {[acetate]/[acetic acid]}

Answer 1

Solution

pH = pk_a + log [CH_3COO^-]/[CH_3COOH] 5 = 4.74 + log 0.25/[CH_3COOH] 5 - 4.74 = 0.26 = log 1.8 log 1.8 = log 0.25/[CH_3COOH] 1.8 = 0.25/[CH_3COOH] [CH_3COOH] = 0.25/1.8 = 0.1388 M Let volume of 2 M potassium acetate = V_1 mL 2 times V_1 = 0.25 times 560 V_1 = 70 mL Let volume of acetic acid = V_2 mL 1 times V_2 = 0.1388 times 560 V_2 = 77.77 mL Volume of water added = 560 - 147.77 = 412.23 mL