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Strong vs Weak Acids
Strong Acids vs Weak Acids
Classification of acids based on their degree of dissociation in water. Strong acids dissociate completely; weak acids establish an equilibrium with their conjugate base.
Comparison Table
| Feature | Strong Acids | Weak Acids |
|---|---|---|
| Dissociation | 100% complete | Partial (equilibrium) |
| Ka Value | Very large (>>1) | Small (<1) |
| [H+] Calculation | [H+] = [acid] | Use Ka expression |
| Conjugate Base | Very weak | Relatively strong |
| pH Calculation | pH = -log[acid] | pH = -log(sqrt(Ka x C)) |
| Common Examples | HCl, HNO3, H2SO4, HBr, HI | CH3COOH, HF, H2CO3 |
| Number Known | Only 7 strong acids | Most acids are weak |
| In Solution | No HA molecules remain | HA, H+, A- all present |
Key Differences
- →Strong acids completely dissociate; weak acids partially dissociate
- →The conjugate base of a strong acid is too weak to accept protons
- →Weak acid pH calculations require the Ka equilibrium expression
- →Diluting a weak acid changes its percent dissociation
- →There are only 7 common strong acids to memorize
When to Use Strong Acids
- ✓HCl, HBr, HI, HNO3, H2SO4, HClO3, HClO4
- ✓When you need complete dissociation
- ✓Simple pH calculations ([H+] = [acid])
- ✓When a very weak conjugate base is needed
When to Use Weak Acids
- ✓Most organic acids (carboxylic acids)
- ✓Buffer solutions (need equilibrium)
- ✓Biological systems (most use weak acids)
- ✓When partial dissociation is desired
Common Confusions
- !Thinking all acids are strong (most are weak)
- !Confusing acid strength with concentration
- !Forgetting that pH depends on both strength AND concentration
- !Not knowing the 7 strong acids (must memorize)
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Common questions about this comparison
The 7 common strong acids are: HCl (hydrochloric), HBr (hydrobromic), HI (hydroiodic), HNO3 (nitric), H2SO4 (sulfuric), HClO3 (chloric), and HClO4 (perchloric). All other common acids are weak acids.
Set up the Ka expression: Ka = [H+][A-]/[HA]. If Ka is very small compared to concentration, use the approximation [H+] = sqrt(Ka x C). Otherwise, use the quadratic formula. Then pH = -log[H+].