Question

QUESTION 3 Which of the following would make the best buffer at pH 5? 0.5 mol L-1 acetic acid (pKa = 4.76) 0.05 mol L-1 carbonic acid (pKa = 6.3) 0.05 mol L-1 acetic acid (pKa = 4.76) 0.5 mol L-1 carbonic acid (pKa = 6.3)

Answer 1

For a pH goal of 5

then get a pKa value nearest to that value

from the list;

best answer sill be acetic acid, since pKa = 4.76

meaning that there will be a good ratio between the weak acid + conjguate base

as shown here:

pH = pKa + log(COO-/COOH)

note that

carbonic acid is too high, therefore ignore it

Also, we will want to add the most possible moles of the acid/conjugate in order to make it very resistant vs. addition of H+ and OH-

then choose

0.5 mol/L of acetic acid + its conjugate base