A buffer is prepared from a diprotic acid H 2 A and its conjugate base, HA ? . K

Question

A buffer is prepared from a diprotic acid H2A and its conjugate base, HA ? .    

Ka1 = 1.38 x 10-4.   Ka2 = 7.24 x 10-7.  Calculate the concentrations of each member of the buffer if pH is 4.15, and the total amount in the system is 0.0482 M?

(b)  (0.4 pt) For the diprotic acid system in part (a), what species (one or two) is/are predominate at each of the pH's below?

            At pH 3           _________      at pH 5            __________    at pH = pKa2 ____________________________

                                    At pH  =  8 __________________

(c)  (0.3 pt) Write the mass balance equation for this diprotic acid system at all pH's.

Explanation / Answer

1) pH = pKa1 + log [HA] / [H2A]

4.15 = -log 1.38 x 10-4 + log [HA] / [H2A]

[HA] / [H2A] = 0.2898

So [HA] = 0.2898 [ H2A]

But given

[HA] + [H2A] = 0.0482

so by solving we get

[H2A] = 0.03736 M

[HA] = 0.01089 M

2) Isoelectric point

Pi =( pKa1 + pKa2 )/2

PI = 5

So isoelctric point is at pH = 7

at pH = 3

dominant species is H2A , HA

at pH = 5

dominant species is HA

at PH = Pka2 = 6.14

dominant species is HA, A-

at PH = 8

dominant species A-

c) H2A ---> HA- + H+

HA- ----> H+ + A-

Mass balance equation is

[H2A] initial - [H2A] - [HA-] - [A-] =0

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