Question

Consider how to prepare a buffer solution with pH = 3.66 (using one of the weak acid/conjugate base systems shown here) by combining 1.00 L of a 0.305-M solution of weak acid with 0.271 M sodium hydroxide.

Weak Acid	Conjugate Base	Ka	pKa
HNO2	NO2-	4.5 x 10-4	3.35
HClO	ClO-	3.5 x 10-8	7.46
HCN	CN-	4.0 x 10-10	9.40

How many L of the sodium hydroxide solution would have to be added to the acid solution of your choice?

L

Answer 1

we should chose the acid whose pKa is closer to required pH

So I chose HNO2/NO2- buffer

Let volume of NaOH added be V L

so, mol of NaOH added = 0.271 M * V L = 0.271*V mol

mol of HNO2 present initially = 0.305 M * 1 L = 0.305 mol

both will react to form NO2-

mol of NO2- formed = 0.271*V mol

mol of HNO2 remaining = 0.305 - 0.271*V

use:

pH = pKa + log {[NO2-]/[HNO2]}

3.66 = 3.35 + log (0.271*V / (0.305 - 0.271*V))

log (0.271*V / (0.305 - 0.271*V)) = 0.31

0.271*V / (0.305 - 0.271*V) = 2.042

0.271*V = 0.623 - 0.553*V

V = 0.756 L

Answer: 0.756 L