To the nearest hundredths, what is the pH at the end of an enzyme-catalyzed reaction if it were carried out in a 0.1 M buffer initially at pH 6.66, and 0.004 M of acid (H+) was produced during the reaction? (The buffer used was a polyprotic acid of the general form: H3A; the three pKas of this polyprotic acid are 2.16, 6.66, and 11.23.)
Solution:
pH matched with pka2 , let acid be H3A hence the buffer would be H2A- and HA2-
We use Henderson equation of buffer
pH = pka2 + log [HA2-]/[H2A-]
6.63 = 6.63 + log [HA2-]/[H2A-]
[[HA2-] =[H2A-] ............(1)
we were mentioned buffer conc is 0.1 M = [H2A-]+[HA2-] .......(2)
by (1) and ( 2) we get [HA2-]=[H2A-] = 0.05 M
now 0.003 M acid is added , acid H+ reacts with HA2- and we get H2A-
H+ (aq) + HA2- (aq) ---> H2A-(aq)
hence [H2A-] = 0.05+0.004 = 0.054 , [HA2-] = 0.05-0.004 = 0.046 M
now pH = 6.63 + log ( 0.046/0.054)
= 6.56
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