Which of the following would be the best buffer at the stated pH?
Since all pH values are 5.1, the best buffer would be one with a pKa closest to 5.1. That would be #2 and #4 whose pKa values are 4.2. So to differentiate these two, I would pick the one with the most buffer capacity, and that would be #4 as it is 0.2M compared to #2 which is 0.05M
mol HCl = 0.015 L x 0.1 mol/L = 0.0015 mol
mole NaOH = 0.05 L x 0.1 mol/L = 0.005 mol
Excess NaOH = 0.005 – 0.0015 = 0.0035 mol OH-
Total volume = 65ml = 0.065L
[OJH-] = 0.0035 mol/0.065 L = 0.0538 M
pOH = 1.27
pH = 12.73