Predicting Reaction Extents
17.5
Last Class
HCl: Strong Acid
(Weak Base)
• 17.6 Types of Acid-Base Reactions
What is the final pH when we add strong, weak acid-base
combinations?
Acetic Acid:
Weak Acid (Weak
Base)
Mixture of an
Acid and a Base
The weak acid-base pairs are in greater abundance at equilibrium
Today
What is the KA of acetic acid if a
solution 0.454 M has a pH of 2.561?
Equation
• 17.7 Calculations
– Given pH and concentration: calculate KA or KB
– Given KA (or KB) and concentration: calculate pH
AcOH, KA =
•
• 17.8 Polyprotic Acids
1
What is the pH of a solution 0.63 M of
HCN KA = 4.0 x 10-10?
Equation
Rule:
if [HCN] > 100 KA
x << [acid]
HCN,, KA =
[H3O+] =
• If the rule is not valid you have to solve the
quadratic equation!
And we want to avoid this (if possible)
possible).
2
Polyprotic acids
Ka
1.8 10-5
1.4 10-3
3.2 10-3
2 10-1
If
Rule: if [[acid]] > 100 Ka
([acid] – x) ~ [acid]
[acid] = 0.5 M
Ka
1.8 10-5
pH using rule
2.52
1.4 10-3
3.2 10-3
2 10-1
1.58
1.41
0.63
quadratic
2.52
1.59
1.40
0.50
Citric acid
Polyprotic acids in life
Tartaric acid
Malic acid
Amphiprotic ions
3
Salt from polyprotic acids: acid or basic
solutions?
Na2HPO4 (aq) ' 2Na+ (aq) + HPO42- (aq)
HPO42- + H2O (l) ' PO43- + H3O+
Ka= 3.6 10-13
HPO42- + H2O (l) ' H2PO43- + OH- Kb= 1.6 10-7
Kb > Ka Æ pH increase (Basic solution)
4