Presentation on theme: "Exp 4C: How Much Acetic Acid is in Vinegar?"— Presentation transcript:
1 Exp 4C: How Much Acetic Acid is in Vinegar? Purpose – To use quantitative analysis and titrations to find the molarity of an acid or base. In this experiment, we shall determine the amount of acid in vinegar.
2 Exp 4C: How Much Acetic Acid is in Vinegar? Quantitative analysisDetermining the specific amount (molarity or concentration) of a particular substance in a sample(qualitative analysis determines the presence w/o specifying the amount)ExamplesCholesterol levels in serumIron in bloodLead in drinking waterNO in airDrug tests for athletesEtc.
3 Exp 4C: How Much Acetic Acid is in Vinegar? TitrationDetermining the amount of a substance by a reaction with another substance with known concentrationDetection of the chemical reaction by some kind of signal, e.g., color change of an indicatorPurpose of the ExperimentDetermine the amount of acetic acid in vinegar by titration with sodium hydroxideCH3COOH + NaOH CH3COONa + H2OH+ + OH- H2ONeutralization ReactionIndicator is phenolphthaleinChanges from colorless (acidic) to pink (basic)
4 Exp 4C: How Much Acetic Acid is in Vinegar? Analysis by TitrationTake a known volume of acetic acidAdd sodium hydroxide of a known concentration in small amountsCH3COOH + NaOH CH3COONa + H2OH+ + OH- H2OAs long as there is more acetic acid than sodium hydroxide, the solution reacts acidicIndicator will be colorless
5 Exp 4C: How Much Acetic Acid is in Vinegar? Analysis by Titration (cont’d)When acetic acid initially in solution equals sodium hydroxide added, you have reached the equivalence point (neutral solution)When more sodium hydroxide than acetic acid is present, the solution turns basicIndicator will become pinkWhen the solution becomes just pink with one drop of NaOH, you have reached the “endpoint” of the titration
6 Point of neutralization An Acid-Base TitrationAddition of base until all acid is neutralizedBaseIndicatorAcidStart of titrationExcess of acidPoint of neutralizationSlight excess of base
7 Point of neutralization An Acid-Base TitrationPoint of neutralizationH+(aq) + OH-(aq)] H2O(l)Point of neutralization (“Equivalence point”)All moles of H+ ions present in the original volume of acid have reacted with an equivalent amount of moles of OH- ions form the base addedmoles of H+ (originally in flask) = moles of OH- (added from buret)
8 Exp 4C: How Much Acetic Acid is in Vinegar? Analysis by Titration (cont’d)Equivalence PointCH3COOH + NaOH CH3COONa + H2OH+ + OH- H2OEvery single ion of H+ from acetic acid has reacted with an OH- ion from sodium hydroxide to form H2OMoles of acid = moles of baseMoles of H+ = moles of OH-Moles of acid = concentrationacid * volumeacid = mol/L * LMoles of base = concentrationbase * volumebase = mol/L * LCalculation of acid-base concentrations for a molar ratio of 1:1Macid * Vacid = Mbase * Vbase if you know the volume of acid, the volume of base in a titration and the concentration of one of the components, you can calculate the concentration of the other
9 Exp 4C: How Much Acetic Acid is in Vinegar? Experimental:Preparation (Different than in lab manual!!! Take notes)Get 50 mL buret and 10 mL pipetMeasure about 30 mL of vinegar in one beaker and ~ 85 mL of NaOH solution, 0.20 M in another beakerPlace buret in clamp and add about 5 mL of NaOH solution using a funnelClean buret by carefully rotating and pouring out the 5-mL NaOH solution into a waste beakerPlace buret back in buret clamp
10 Exp 4C: How Much Acetic Acid is in Vinegar? Experimental: Trial Titration aka Rapid Titration (WHY?)Put 5.0 mL of vinegar in a clean 125 mL Erlenmeyer flaskAdd ~ 20 mL dH2OAdd 2 drops of phenolphthaleinFill the buret with 50-mL of NaOHRecord the molarity of NaOHUse a funnel to fill the buret, not over the 0.0 lineRecord the initial volume (SIG FIGS!!!!)Put the tip of the buret in the Erlenmeyer, and a piece of white paper underneath the flaskAdd the NaOH solution in 1-mL increments to the acetic acid while swirlingNote the color after swirlingWhen you get close to the equivalence point, the solution will briefly turn pink and then colorless againAdd smaller amounts until the solution stays pink for about 30 secondsRECORD VOLUME OF NaOH ADDED (final reading – initial reading)
11 Exp 4C: How Much Acetic Acid is in Vinegar? Experimental: Exact TitrationPut 5.0 mL of vinegar in a clean 125 mL Erlenmeyer flaskAdd ~ 20 mL dH2OAdd 2 drops of phenolphthaleinFill the buret with 50-mL of NaOHRecord the molarity of NaOHUse a funnel to fill the buret, not over the top lineRecord the exact initial volumePut the tip of the buret in the Erlenmeyer, and a piece of white filter paper underneath the flaskSubtract about 1-2 mL from the volume determined in the trial titrationRapidly add the remainder to the flaskRinse the wall of the flask with dH2O (Why?)Continue adding NaOH drop-by-drop until the solution has a barely visible pink color that does not go away upon swirling
12 Exp 4C: How Much Acetic Acid is in Vinegar? Experimental: Exact Titration (continued)Record the volume of the buretThe volume added is Vfinal - VinitialRepeat the procedureCalculate the molarity for each procedure using the formula for the titration.The difference must be not more than 2%.Calculate the molarity of the vinegar
13 For next week and after: Due March 26:1. Results and post-lab for Exp 4C2. No Pre-lab dueApril 2: Long Quiz (See Study Guide given in class, to be posted):Exp. 1A, 3A, 3B, 4A, 4Ba. Know the purpose and general techniques for eachb. Be able to interpret resultsc. Understand and explain safety precautionsd. Study the pre- and post-lab questionse. Know how to do all calculations