Experiment 3_ Exploring the Behavior of Acids

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Feb 20, 2024

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1 9/25/21 Experiment 3: Exploring the Behavior of Acids Exploring the behavior of an acid/conjugate base pair to understand the “protonated” and “deprotonated” forms of a molecule. Goals : Perform titrations for an unknown organic acid, plot data on a pH curve and take an IR and melting point of your carboxylic acid. Reagent Table : Compound Molecular Weight (g/mol) Density (g/mL) Used Safety Trans-cinnamic acid (C H O ) ₉ ₈ ₂ (most accurate unknown compound) 148 1.25 0.14 g Irritant, causes skin and eye irritation Malonic acid (C H O ) (2nd ₃ ₄ ₄ best unknown compound) 104 1.63 0.14 g Corrosive, Irritant; harmful if swallowed, causes skin and eye irritation

2 Sodium Hydroxide (NaOH) 39.997 2.13 Setup 15 mL in burette ~1.4 mL used in titration Corrosive, causes severe skin burns and eye damage Hydrochloric Acid (HCl) 36.46 1.18 1 mL Corrosive, causes severe skin burns and eye damage, toxic if inhaled Procedure : 1. Obtain a pH meter: a. Remove the electrode from the saturated KCl solution and place the plastic vial in a safe spot to prevent any spilling. b. Then, open the electrode ring on the electrode. c. Lastly, calibrate the pH meter if necessary (follow directions on the back of the pH meter) and rest the electrode in a beaker of water. 2. Weight out approximately 100-200 mg (0.1-0.2g) of your acid and record the exact mass that you will be using in the reagent table. 3. Dissolve the acid in approximately 50 mL of deionized water in a 100-150 mL beaker/flask. Dissolving the acid completely may take time since organic acids may not dissolve well in water which is why you should titrate slowly. 4. Place a stirring bar in the acid solution and place on a magnetic stirrer with the heat off. Most of the solid should disappear. If not, you must titrate very slowly. 5. Put the electrode in the stirring solution, making sure to not hit the end of the electrode with the stir bar. 6. Record the initial pH in your lab notebook. a. If the pH is greater than 2, add about 1 mL of 1M HCl with a poly pipette to ensure that the acid is fully protonated. Record the pH again.

3 7. Set up the titration apparatus by first rinsing a burette with deionized water. Make sure that the stopcock works before continuing. 8. Fill the burette with approximately 20 mL of the 1M NaOH - you will not use all of this 9. Titration: Adding 0.2-0.5 mL increments of 1M NaOH, record the pH every time you make an addition. Continue doing so until you notice that the pH is increasing rapidly. Keep adding NaOH until the pH is above 10. 10. Titration curves: Plot your graph with pH on the y-axis and mL of NaOH on the x-axis. Use a lab computer or sketch accurately in your lab notebook. 11. Return your pH meter to the plastic container with saturated KCl and tighten the cap. Close the electrode ring. 12. Take an IR and a melting point for your carboxylic acid. Data : Melting point: 130-136℃ → 136℃ is the melting point Initial pH: 3.63 pH after adding 1 mL HCl = 1.59 Titration Data Table : Number of additions Amount of NaOH (mL) pH 0 0 1.59 1 0.3 1.70 2 0.3 1.81 3 0.2 2.07 4 0.2 2.47 5 0.2 3.99 6 0.1 8.87 7 0.1 10.04 Amount of NaOH (mL) pH Buffer region (pH 1.70-3.99): 1 mL NaOH Equivalence point (pKa=pH) pKa=2.07 Titration Curve of Unknown Compound

4 Calculations : Number of moles of NaOH required to move across the buffer zone: 1 mL = 0.001 L 0.001 L× 1 mol 1 L = 0.001 mol = 1.00 × 10 − 3 mol Number of moles of the acid: 1.4 g× 1 mol 135 g = 0.0104 mol = 1.04 × 10 − 3 mol Ratio of moles of the acid to moles of NaOH: 1.04 × 10 − 3 mol 1.00 × 10 − 3 mol = 1.04 mol 1 mol ≈ 1:1 molar ratio Number of moles of trans-cinnamic acid: 1.4 g× 1 mol 148 g = 0.000946 mol = 9.46 × 10 − 4 mol Ratio of moles of trans-cinnamic acid to moles of NaOH: 9.46 × 10 − 4 mol 1.00 × 10 − 3 mol = 0.946 mol 1 mol ≈ 1:1 molar ratio IR spectra from lab :

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