Question

Buffers can also be basic. Would a solution that is 0.2M in ammonia and 0.2M in ammonium chloride be a buffer?Explain. Could you make a brief drawing showing the composition of this solution. Write the henderson-hasselbach equation for a basic buffer and calculate the pH.

Answer 1

Ammonia (NH₃) is a Bronsted base and can accept a proton to form the conjugate acid, NH₄⁺. Conversely, NH₄⁺ is a Bronsted acid and can donate a proton to form NH₃. Therefore, NH₃/NH₄⁺ can act as a weak base/conjugate acid and hence a combination of NH₃/NH₄⁺ (as NH₄Cl) can act as a basic buffer. The pK_b (base dissociation constant) of the weak base, NH₃ is 4.75.

The Henderson-Hasslebach equation gives the pOH for a weak base. The pOH for a buffer o 0.2 M NH₃ and 0.2 M NH₄Cl is given as

pOH = pK_b + log [NH₄Cl]/[NH₃] = 4.75 + log (0.2 M)/(0.2 M) = 4.75 + log (1.00) = 4.75

Employ the relation pH + pOH = 14.00 to find the pH of the buffer. The pH is pH = 14.00 – pOH = 14.00 – 4.75 = 9.25 (ans).