Question

Consider a buffer system consisting of Benzoic acid (pKa = 4.20) and sodium benzoate,

a) Would this buffer system be an appropriate choice to maintain a pH of 8.5? ues or no

b) To produce a buffer with a pH = 4.55, what mass of sodium benzoate (FM = 104.10) must be dissolved in 500 mL of 0.65 M benzoic acid?

c) What is the resulting pH if 45-mL of 0.08-M HCl is added to 250 mL your buffer?

Answer 1

a) the answer is no, it's common that the buffers contain the same concentration of acid and base, in this case the pH of the buffer is giving by the pKa, it could vary in +/- 1 depending of the concentration, but no more than that because it would break the equilibrium and therefore there will be no buffer.

Notice that the pH of the buffer change from 4.55 to 4.53, and this small variation is due to the buffer effect, and of course the solution is a little more acid because we are adding a strong acid.