Chapter 17 -- Sample problem
Ranking a group of acids and bases in order of increasing pH
Rank each catagory from low pH to high pH.
Remember: low pH is more acid or
less basic!
- Consider relative strength andconcentration
- Strong acids/bases dissociate fully, weak do not.
- If the Ka of a weak acid is greater than
10-7 the Kb of it's conjugate base will be
less than 10-7.
Catagories
| Acidic | Basic | Neutral |
| 0.2 M HNO3 | 0.05 M NH3 | 0.1 M NaCl
|
| 0.05 M CH3COOH | 0.03 M Ca(OH)2 |
|
| 0.05 M NH4Cl | 0.05 M CH3COONa
|
|
| 0.1 M H2SO4 | 0.05 M NaOH |
|
| 0.05M CF3COOH | |
|
Details
Acids
- 0.2 M HNO3: strong acid--[H+]=0.2 M
- 0.05 M CH3COOH: weak acid (Ka
~10-5)--[H+] much less than 0.05 M
- 0.05 M NH4Cl: weak acid
(Ka~10-10)--[H+]
very much less than 0.05 M
- 0.1 M H2SO4: strong acid for first
dissociation (contributes 0.1 M to overall [H+]), weak
acid for second dissociation (contributes less than 0.1 M to overall
[H+]--[H+] will be greater than 0.1 M
and less than 0.2 M
- 0.05 M CF3COOH: weak acid--stronger than
CH3COOH because of the electron withdrawing properties of
the flourine atoms--[H+] will be less than 0.05 M but will
be greater than the [H+] in CH3COOH
Bases
- 0.05 M NH3: weak base (Kb
~10-5)--[OH-] less than 0.05 M
- 0.03 Ca(OH)2: strong base--[OH-]=0.06 M.
- 0.05 M CH3COONa: weak base (Kb
~10-10)--[OH-] very much less than 0.05 M
- 0.05 NaOH: strong base--[OH-]=0.05 M
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