Overview
This lecture covers titration curve calculations for a weak base (CH₃NH₂) titrated with a strong acid (HCl), explaining each step and the different regions of the titration curve.
Titration Curve Basics
- A titration curve shows pH changes as an acid or base is added to a solution.
- The curve highlights specific pH points of interest during titration.
- Equivalence point: when moles of acid equal moles of base—solution is neutralized.
Initial pH Calculation
- Initial pH is calculated before any titrant is added; focus on what's in the flask.
- For 15 mL of 0.1 M CH₃NH₂, use weak base pH calculation:
- ([H_3O^+] = \sqrt{K_a \times K_w / [\text{base}]})
- pH = -log(([H_3O^+])) gives initial pH; example result: 11.8.
Halfway to Equivalence Point
- Halfway point: enough acid added to neutralize half the base.
- At this point, ([\text{base}] = [\text{conjugate acid}]) and a buffer is formed.
- pH at halfway = pKa (here, 10.64); no calculation needed.
Buffer Region Calculation (e.g., 6 mL HCl Added)
- Add small amounts of strong acid (HCl) to weak base forms a buffer system.
- Calculate moles of base and acid before and after reaction.
- Use Henderson-Hasselbalch: (\mathrm{pH} = \mathrm{p}K_a + \log(\text{base}/\text{acid})).
- Example: pH = 10.8 after 6 mL HCl added.
Equivalence Point Calculation
- Equivalence point: all base neutralized; only conjugate acid remains.
- Find moles and concentration of new weak acid (CH₃NH₃⁺) using total volume.
- Calculate ([H_3O^+]) for weak acid: ([H_3O^+] = \sqrt{K_a \times [\text{acid}]}).
- Example pH at equivalence: 5.97 (always <7 for weak base/strong acid titrations).
After Equivalence Point (Excess Acid Added)
- Beyond equivalence (e.g., 25 mL HCl), only excess strong acid determines pH.
- Find concentration of excess HCl, accounting for total solution volume.
- pH = -log([H₃O⁺] from excess acid); example: pH = 1.6.
Titration Curve Features
- Initial pH is high (weak base).
- Buffer region around pKa (pH = pKa ± ~1).
- Sharp drop at equivalence; pH is acidic due to weak acid.
- Very low pH after excess strong acid is added.
Key Terms & Definitions
- Titration curve — Graph showing how pH changes as titrant is added.
- Equivalence point — Moles of acid equal moles of base; reaction complete.
- Buffer region — Area where a weak acid and its conjugate base resist pH changes.
- Henderson-Hasselbalch equation — Calculates buffer pH: (\mathrm{pH} = \mathrm{p}K_a + \log(\text{base}/\text{acid})).
- pKa — Negative log of the acid dissociation constant; characteristic of each weak acid.
Action Items / Next Steps
- Review buffer and weak acid/base pH calculations.
- Practice similar titration problems, especially with varying concentrations.
- Watch for the next video (part 2) covering cases with different titrant concentrations.