Wednesday
[2]
1.
Quiz 8
Chem 265
2012
Starting with the fully protonated species, write the stepwise acid dissociation
reactions of glutamic acid. Be sure to remove the protons in the correct order.
Which species is the neutral molecule that we call glutamic acid?
NH3+
∣
CHCH2CH2CO2H
∣
CO2H
NH3+
∣
CHCH2CH2CO2H
∣
CO2NH3+
∣
CHCH2CH2CO2∣
CO2NH2
∣
CHCH2CH2CO2∣
CO2-
NH2
∣
CHCH2CH2CO2H
This is the neutral species as we write the formula.
∣
CO2H
It would exist in solution as the zwitterion:
NH3 +
∣
CHCH2CH2CO2H
∣
CO2-
[5]
2.
A 0.020 M solution was prepared by dissolving dipotassium cysteine, K2C, in
water. Then 30.0 mL of this solution was titrated with 0.040 M HClO4.
Calculate the pH at the first equivalence point.
Ka1 = 2 x 10-2
Ka2 = 4.4 x 10-9
Ka3 = 1.82 x 10-11
First need concentration at the equivalence point.
Analyte
Titrant
Product
Reaction
C2H+
HCStoichiometry
1
1
1
Molarity
0.020
0.040
Volume
30.0 mL
15.0 mL
45.0 mL
Initial moles
0.6 mmol
0.600 mmol
Final moles
0.600 mmol
[KHC] = 0.60/45 = .013 M
Relevant Kas: Ka2 = 4.4 x 10-9
[𝐻 + ] = √
Ka3 = 1.82 x 10-11
𝐾𝑎2 𝐾𝑎3 + 𝐾𝑎2 𝐾𝑤
𝐾2 + 𝐹
= 2.86 x 10-10 so pH = 9.54
If you use the simple equation, get 9.52 worth 3
[3]
3.
What is the concentration of M2- in 0.100 M H2M?
Ka1 = 1.42 x 10-3 Ka2 = 2.01 x 10-6
[M2-] = Ka2 = 2.01 x 10-6