1. Chem. 1B – 9/15 Lecture

2. Announcements I Lab Report for Lab #1 due Monday/Tuesday
Second Mastering Set due Tuesday (should cover material to do most problems today)
Exam #1
Two weeks from today (9/29)
Covers material covered through next Thurs. (including buffers)

3. Announcements I Today’s Lecture
[H+] and pH in strong acid and weak acid solutions
% ionization of weak and strong acids
Base solutions + solving equilibrium problems
Acid-base properties of ions and salts

4. Chem 1B – Aqueous Chemistry Equilibrium Problems Involving Acids
Strong Acids (large Ka values)
Example: Determine the pH of a M HCl solution
We could set up an ICE table, but with strong acids, we assume 100% dissociation
HCl(aq) → H+(aq) + Cl- (aq)
So [H+] = [HCl(aq)]o = M
pH = -log[H+] = 1.85
Note: this approach works as long as [H+] from HA is greater than [H+] from H2O

5. Chem 1B – Aqueous Chemistry Equilibrium Problems Involving Acids
Weak Acids (Ka values < 1)
Example: Determine the pH of HCHO2 (formic acid, Ka = 1.8 x 10-4) solution initially made to be 0.20 M.

6. Chem 1B – Aqueous Chemistry Acid Reactions and Equilibrium
% Ionization of Weak Acids (or % dissociation)
Given by [A-]*100/[HA]all forms
Show example from the 0.20 M formic acid problem covered last time

7. Chem 1B – Aqueous Chemistry Acid Reactions and Equilibrium
Generalities about % Ionization
The greater Ka, the greater % ionized (example for 0.05 M)
The lower the concentration, the greater the % ionized (example for formic acid)
Acid
HNO2
HCHO2
HC2H3O2
HClO
HCN Ka
4.60E-04
1.80E-04
1.80E-05
2.90E-09
4.90E-10
%ionized
9.14
5.82
1.88
0.02
0.01
[HA]o
0.5 M
0.05 M
5E-03
5E-04
5E-05
%ionized
1.88
5.82
17.26
44.64
81.53

8. Chem 1B – Aqueous Chemistry Acid Reactions and Equilibrium
Mixtures of Acids
Example: what is the pH of a raindrop if its sources of acid are from H2CO3 (from CO2 gas) and NH4HSO4?
This is one of the few general chemistry texts covering this
However, in general we cannot solve these problems using the approach covered in this class as the approach can fail when more than one equilibrium occurs
In selected cases (where concentrations are similar but Ka values are very different), it is possible to solve these problems

9. Chem 1B – Aqueous Chemistry Acid Reactions and Equilibrium
Mixtures of Acids – back to example:
What is the pH of a raindrop if [H2CO3]o = 1.4 x 10-5 and (from CO2 gas) and [NH4HSO4]o = 1.0 x 10-4 M?
In this case there are 4 sources of H+: HSO4- (Ka2 = 0.012), H2CO3 (Ka1 = 4.3 x 10-7), NH4+ (Ka = 5.7 x 10-10), and HCO3- (Ka1 = 5.6 x 10-11) (actually 5 if we include water)
Because the Ka value for HSO4- is much greater than the others, we can assume that determines the pH
Show ICE approach for HSO4-
Once we know [H+], we can determine the concentration of other species (e.g. [HCO3-])

10. Chem 1B – Aqueous Chemistry Acid Reactions and Equilibrium
Other Types of Problems
An unknown acid is prepared so its concentration is M. Its pH is measured and found to be What is the Ka of this acid? What is its % ionization?

11. Chem 1B – Aqueous Chemistry Base Reactions and Equilibrium
Strong Bases
Most Strong Bases are Metal Hydroxides (MOH)
Since metal hydroxides are ionic compounds, they totally dissociate
Some are not very soluble (e.g. Zn(OH)2), but considered strong bases because what does dissolve 100% dissociates
Example: Calculate the pH if we have an aqueous Ba(OH)2 solution prepared to be M.

12. Chem 1B – Aqueous Chemistry Base Reactions and Equilibrium
Weak Bases
Generic Reaction:
B(aq) + H2O(l) ↔ BH+(aq) + OH-(aq) (B = base)
Kb = [BH+(aq)][OH-(aq)]/[B(aq)] (why no H2O?)
Stronger weak base has larger Kb value
Examples:
NH3 (ammonia)
CH3NH2 (methylamine)
C5H5N (pyridine)
Most of these examples smell bad (e.g. tuna fish)
Molecule is volatile but cation form is not
Le Châtelier’s Principle Question: Explain why adding acid (mustard) to tuna fish reduces its smell

13. Chem 1B – Aqueous Chemistry Base Reactions and Equilibrium
Weak Bases – Example Problem
Example: Determine pH of a NH2OH (hydroxylamine) solution (Kb = 9.1 x 10-9) initially at M

14. Chem 1B – Aqueous Chemistry Acid-Base Properties of Ions
In looking at acids and bases, we have restricted ourselves to uncharged compounds
However, the ions produced by the acid and base reactions are acids and bases
Example: Acetate (C2H3O2-) is the conjugate base of acetic acid.
Is it an acid or a base?
What is its Kb if Ka(HC2H3O2) = 1.8 x 10-5?
(Hint: combine the acid reaction with the water protolysis reaction to determine Kb)
How is conjugate base strength related to acid strength?

15. Chem 1B – Aqueous Chemistry Acid-Base Properties of Ions
In general, cations will be acidic (conjugate acids of bases or water/hydroxide complexing metals) or neutral
Group I and II metals complex poorly with hydroxide and can be considered neutral
Anions will be basic (conjugate bases of weak acids), or neutral (conjugate bases of strong acids)
“Hydrogen anions” can be acidic (HSO4-) or basic (HCO3-)
For salts, must examine both cation and anion

16. Chem 1B – Aqueous Chemistry Acid-Base Properties of Ions
Example Question: Determine if the ionic compounds are acidic or basic in the following examples:
NaCl
NH4Cl
NaC2H3O2
Fe(NO3)3
NH4CN

17. Chem 1B – Aqueous Chemistry Polyprotic Acids
Generic Example: H2A – has two protons that can be lost through acid reactions (diprotic)
Some Examples:
H2SO4 (sulfuric – first H+ loss is strong acid)
H2SO3 (sulfurous)
H2CO3 (carbonic)
H3PO4 (phosphoric – triprotic)
Reaction of generic diprotic example
H2A(aq) ↔ H+(aq) + HA-(aq) K = Ka1
HA- (aq) ↔ H+(aq) + A2-(aq) K = Ka2

18. Chem 1B – Aqueous Chemistry Some Practice
Which solution will have a greater fraction of ionization? 0.10 M HClO vs M HF
Ka(HClO) = 2.9 x Ka(HF) = 3.5 x 10-4
An unknown base is dissolved in water so that its initial molarity is M. The pH is measured and found to be What is its Kb value?
The Kb for NH3 is 1.76 x What is the pH of a solution initially made to 0.10 M NH4Cl?

19. Chem 1B – Aqueous Chemistry Polyprotic Acids – in Problems
Solving polyprotic acid problems can be challenging (the concentrations of the products from the first reaction affect the equilibrium in the second reaction)
To simplify the problem, we assume the two reactions occur independently (valid if Ka1 >> Ka2)
Example Problem: calculate [H2CO3], [HCO3-], pH, and [CO32-] for a 1.0 x 10-3 M solution of H2CO3