Overview
This lecture details Lab 8: Determining the acid dissociation constant (Ka) of an unknown weak acid using titration with a strong base (NaOH) and a pH meter.
Experimental Setup
- Use known concentration of sodium hydroxide (NaOH) as the titrant and an unknown weak acid as the analyte.
- Measure pH changes with a pH meter as NaOH is gradually added to the acid solution.
- Initial pH readings of the unknown acid are below 7, confirming its acidity.
- The buret must be filled accurately to zero, ensuring no air bubbles.
- 10 mL of unknown weak acid is prepared in a beaker; a stir bar is added for homogeneity.
Data Collection and Titration Process
- NaOH is added in small increments (0.5 mL at first, larger increments later), and pH is recorded after each addition.
- Plot pH (y-axis) vs. cumulative volume of NaOH added (x-axis) to produce a titration curve.
- The pH increases slowly at first, reaches a plateau, then spikes rapidly at the equivalence point.
- The equivalence point for weak acid/strong base titration occurs above pH 7 (typically >9).
- In the first trial, endpoint is observed at around 10.5 mL added NaOH and pH >9.
- In the second trial, larger increments are used initially, then smaller ones near the equivalence point to precisely identify the pH jump.
Analysis and Results
- The sudden jump in pH between ~9.5 and 10.2 mL added NaOH marks the equivalence point.
- Endpoint confirmation: pH meter reading above 9 and color change of a pH indicator.
- Second trial refines accuracy by narrowing drop size near the spike.
Key Terms & Definitions
- Titration — Gradual addition of one solution to another to determine concentration.
- Equivalence Point — Point where moles of titrant equal moles of analyte; for weak acid with strong base, pH >7.
- Ka (Acid Dissociation Constant) — Measures the strength of a weak acid in solution.
- pH Meter — Instrument measuring the acidity/alkalinity (pH) of a solution.
- Buret — Precision glassware for dispensing accurate volumes of liquid, read from top (0 mL) downward.
Action Items / Next Steps
- Plot the titration curve for both trials (pH vs. volume NaOH).
- Determine the exact volume at the equivalence point for Ka calculations.
- Record all pH values and volumes clearly in your lab notebook.
- Review lab manual example graph for reference before analysis.