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CHM 320 Lecture 18 Chap 11
Fractional Composition Equations Why determine fractional compositions?•^
Fractional composition clearly show the dominant and minorspecies in solution.
-^
Fractional compositions provide a way to know what reactionsare occurring, and what reactions are relatively unimportant.
-^
Fractional compositions make really cool plots. Monoprotic Systems For an acid -
α
HA
= [HA] / F
α
A-
= [A
-^ ] / F
For a base -
α
b^
= [B] / F
α
BH+
= [BH
+^ ] / F
F = Formal concentration
1.2^1 0.8 0.6 0.4 0.2^00
1
2
3
4
5
6
7
8
pH
fractional composition
alpha (HA)
alpha (A-)
pKa
CHM 320 Lecture 18 Chap 11
Diprotic Systems For an acid -
α
H2A
= [H
A] / F 2
α
HA-
= [HA
-^ ] / F
α
A2-
= [A
] / F
F = Formal concentration
1.2^1 0.8 0.6 0.4 0.2^00
1
2
3
4
5
6
7
8
pH
fractional composition
alph H2A
alph HA-
alph A-
pKa
pKa
CHM 320 Lecture 18 Chap 11
Biochemical terms Isoionic pH : pH of a solution where the pure, neutral
polyprotic acid has been dissolved.
Isoelectric pH : pH of a solution when the average charge
on the polyprotic acid is 0.
The isoelectric pH is useful for separating various
polyprotic acids from one another. If you put these acidsin a electric field and a pH gradient, they will focusthemselves in a region of the pH corresponding to theirisoelectric pH (they do not move toward the positive ornegative pole).
CHM 320 Lecture 18 Chap 11
Diprotic Buffers – treated similar to a monoprotic buffer. Youmust know what the desired pH and the appropriate pKa are. pH = pKa + log [base]/[acid]
CHM 320 Lecture 18 Chap 11
Calculate the pH of a 0.10 M solution of each amino acid. NH
2 C O CH
2 CH
2
H
N 3
+^ CHC-O
O
glutamine
S
2
H
N 3
+^ CHC-O
O
cysteine
H
N 2
NH
2
C NH
2 CH
2 CH
2 CH
2
H
N 3
+^ CHC-O
O
arginine
CHM 320 Lecture 18 Chap 11
Draw the structure and the fractional composition of the principalspecies of 1,3-dihydroxybenzene at the following pH values:
(pK
a
pK
a
pH 9 pH 11
