Question

You need to prepare a phosphoric acid buffer that has a buffer concentration of 0.1 M, a pH of 6.65, and taking in to account activity. First, ignoring ativiy, calculate [A-] and [HA] needed to prepare the buffer using the Henderson-Hasselbalch equation and the fact that [buffer]=0.1 M. (Hint: you have two equations and two unknowns). (pKa1=2.15, pKa2=7.09, pKa3=12.32)

Please show your work and explain your steps.

Answer 1

p^H = p^K_a + log ([Base]/[Acid]) - Henderson-Hasselbalch equation

log ([Base]/[Acid]) = p^H - p^K_a

=  p^H - p^K_a2 (we have to choose the pKa which is close to the p^H of the buffer)

= 6.65-7.09

= -0.44

[Base]/[Acid] = 0.3631

The molarity of the buffer is the sum of the molarities of the acid and conjugate base

[buffer] =  [Base] + [Acid] = 0.1M

  [Acid] = 0.1 - [Base] by substituting this value in above ratio

[Base]/(0.1-[Base]) = 0.3631

[Base] = 0.3631(0.1-[Base])

[Base] = 0.0266 M

[Acid] = 0.1-0.0266

= 0.0734 M