Question

a buffer solution is prepared by mixing 45.0 mL of 0.100M NH3 are mixed with 25.0 mL of 0.200M NH4Cl. calculate the pH of the solution and, with the help of appropriate chemical equations, describe how this buffer works. Include some comments about its capacity for absorbtion of strong acid and strong base.

Answer 1

Buffer NH3/NH4Cl

This is a weak base/conjugate acid type of buffer

Using Hendersen-Hasselbalck equation,

pH = pKa + log(base/acid)

pKa of NH4+ = 9.25

So,

pH = 9.25 + log(0.1 M x 45 ml/0.2 M x 25 ml)

      = 9.20

The buffer works by the equation,

NH3 + HCl <==> NH4Cl

So when strong acid HCl is added, more of free NH3 reacts to form NH4Cl

When strong base NaOH is added, it reacts with free HCl present in the system, so the equilibrium shifts to the left to generate more NH3 from NH4Cl. This is according to the LeChatellier's principle.