Exercise 2, Using the values of Ka given below Formula pKa Acid Formula Acid Ace
ID: 1027935 • Letter: E
Question
Exercise 2, Using the values of Ka given below Formula pKa Acid Formula Acid Acetic Hypobromous HOBr2.8 x 10 HOCI2.9 x 10 2.3 x 10 H,CO HCIO2 Hypoiodous H Oxalic = 4.3 × 10- Carbonic Chlorous Cyanice 1.1 × 10 2.0 x 10 18 x 10 49×1010 3.5 x 104 HC,04 | Sulfurous -so, HSO Kal-1.7x i0° Formic HCOOH Hydrocyaniec HCN Hydrofluoric HF For each p: Pick the best two pairs of compounds that could be mixed to create a buffer of the desired pH (Each mix must contain two complete neutral compounds, NOT JUST IONS!) - State the base/acid ratio that would be needed. - For each pH, note the pair that would create the best buffer a) pH = 3.50 Acid B/A Ratio Base Best? Mix# 1 Mix #2 b) pH 1.20 Acid Base B/A Ratio Best? Mix#1 Mix #2 c) pH = 9.00 Acid Base B/A Ratio Mix#1 Mix #2Explanation / Answer
Determine the pKa values by using the expression pKa = -log Ka.
Acid
Formula
Ka
pKa
Acid
Formula
Ka
pKa
Acetic
CH3COOH
1.8*10-5
4.74
Hyprbromous
HOBr
2.8*10-9
8.55
Benzoic
C6H5COOH
6.5*10-5
4.19
Hypochlorous
HOCl
2.9*10-8
7.54
Carbonic
H2CO3
Ka1 = 4.3*10-7
pKa1 = 6.37
Hypoiodous
HOI
2.3*10-11
10.64
Chlorous
HClO2
1.1*10-2
1.96
Oxalic
H2C2O4
Ka1 = 5.9*10-2
pKa1 = 1.23
Cyanic
HOCN
2.0*10-4
3.70
HC2O4-
Ka2 = 6.4*10-5
pKa2 = 4.19
Formic
HCOOH
1.8*10-4
3.74
Sulfurous
H2SO3
Ka1 = 1.7*10-2
pKa1 = 1.77
Hydrocyanic
HCN
4.9*10-10
9.31
HSO3-
Ka1 = 6.4*10-8
pKa2 = 7.19
Hydrofluoric
HF
3.5*10-4
3.45
A buffer shows its buffering action best when the pKa of the buffer is close to the desired pH. For practical purposes, we must have (pH – 1) < pKa < (pH + 1).
The B/A ratio in the problem is obtain by using the Henderson-Hasslebach equation as
pH = pKa + log B/A
a) pH = 3.50
Acid
Base
B/A ratio
Best?
Mix #1
HF
Sodium fluoride, NaF
3.50 = 3.45 + log B/A
====> log B/A = 0.05
====> B/A = anitlog (0.05) = 1.12
The mix #1 is the best since the pKa of the buffer is closer to the desired pH.
Mix #2
HOCN
Sodium cyante, NaOCN
3.50 = 3.70 + log B/A
====> log B/A = -0.20
====> B/A = antilog (-0.20) = 0.63
b) pH = 1.20
Acid
Base
B/A ratio
Best?
Mix #1
H2C2O4
Sodium hydrogen oxalate, NaHC2O4
1.20 = 1.23 + log B/A
====> log B/A = -0.03
====> B/A = anitlog (-0.03) = 0.93
The mix #1 is the best since the pKa of the buffer is closer to the desired pH.
Mix #2
H2SO3
Sodium hydrogen sulfite, NaHSO3
1.20 = 1.77 + log B/A
====> log B/A = -0.57
====> B/A = antilog (-0.57) = 0.27
c) pH = 9.00
Acid
Base
B/A ratio
Best?
Mix #1
HCN
Sodium cyanide, NaCN
9.00 = 9.31 + log B/A
====> log B/A = -0.31
====> B/A = anitlog (-0.31) = 0.49
The mix #1 is the best since the pKa of the buffer is closer to the desired pH.
Mix #2
H2SO3
Sodium hydrogen sulfite, NaHSO3
9.00 = 8.55 + log B/A
====> log B/A = 0.45
====> B/A = antilog (0.45) = 2.82
Acid
Formula
Ka
pKa
Acid
Formula
Ka
pKa
Acetic
CH3COOH
1.8*10-5
4.74
Hyprbromous
HOBr
2.8*10-9
8.55
Benzoic
C6H5COOH
6.5*10-5
4.19
Hypochlorous
HOCl
2.9*10-8
7.54
Carbonic
H2CO3
Ka1 = 4.3*10-7
pKa1 = 6.37
Hypoiodous
HOI
2.3*10-11
10.64
Chlorous
HClO2
1.1*10-2
1.96
Oxalic
H2C2O4
Ka1 = 5.9*10-2
pKa1 = 1.23
Cyanic
HOCN
2.0*10-4
3.70
HC2O4-
Ka2 = 6.4*10-5
pKa2 = 4.19
Formic
HCOOH
1.8*10-4
3.74
Sulfurous
H2SO3
Ka1 = 1.7*10-2
pKa1 = 1.77
Hydrocyanic
HCN
4.9*10-10
9.31
HSO3-
Ka1 = 6.4*10-8
pKa2 = 7.19
Hydrofluoric
HF
3.5*10-4
3.45
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.