Determination of an Unknown Substance by Titration Larry Sepulveda

1. Determination of an Unknown Substance by Titration Larry Sepulveda Discussion ABSTRACT RESULTS • What did we know about the starting materials? • We know concentration (a form of density), which is weight versus volume. • Concentrations of different species: • [KOH] =mol L-1 = (mass)(molar mass)-1(Lsoln)-1=(4.105g KOH ) (39.10 g/mol)-1 (1 L) = 1.05 M • [Unknown] = 5.873g (60.01 g/mol)-1(1L)-1 = 0.09788 M • [HOAc] = 5.800g (60.01 g/mol)-1(1L)-1 = 0.09651M • What did we know about the finishing materials? • We know that the finishing solution has become more basic according to its change in pH. The higher the pH, the more basic a solution is. We also know the acid dissociation constant1 which helps us calculate the pH. • Acetic acid dissociation constant (Ka ) and pH Calculation: • Ka = [OAc-][H+]/[HOAc] = x2 / (0.09651-x) ≈ x2 / 0.09651 • So x = [H+] = (Ka x 0.09651)1/2 = (1.78x10-5 x0.09651)1/2 = 0.001311 M • And therefore pH = -log 0.001311 = 2.88 • Using the same concentration of base and acid, reproduction of the experiment with acetic acid should yield similar results for Ka: • Ka = [OAc-][H+]/[HOAc] = [OAc-](10-pH)/[HOAc] = (10-2.88)2 /[HOAc] =1.9x10-5 • HOAc pH ≈ unknown pH • But how did we account for error? • See table in “Results” section. • Objective: To determine the identity and industrial use of an unknown substance and then reproduce the results. • Methods: gravimetric analysis and acid/base titration • Major Findings: • - The concentration of the dissolved substance in KOH and solutions was found to be 0.09877 with a standard deviation of 32 ppt. • - The substance was identified after analysis and reproduction to be acetic acid (H3CCOOH). INTRODUCTION • Do you know what’s in your salad? • The food industry uses acetic acid to make vinegars. Our assigned unknown substance is suspected to be this acid. Its chemical formula is H3C-COOH. • Why do we suspect it to be acetic acid? • We used gravimetric analysis, a way to inventory where mass ends up in a reaction, to come up with this hypothesis. First an experiment was demonstrated to us using our unknown. Then, with a list of possible reactions, gravimetric analysis calculations were carried out (see the concentration calculation in the discussion section for an example). • How do we verify that it is acetic acid? • Sample Analysis Method: Titration is a method of acid/base analysis. A weakacid or base is treated with a strong base or acid respectively to determine different properties of the weakspecies. • Acetic acid (or simply, HOAc) reacts with a strong base in the following way: • HOAc+ OH-  -OAc+ H2O • The acetate ion (-OAc)then undergoes the following equilibrium reaction: • -OAc+ H2 O OHAc+ OH- • Every acid has a specific numerical value (Ka) that describes how strong it is. Every acid also has a number (pH) that describes its acidity after titration (equivalence volume, or Veq). If the textbook values match our experimental values, then our substance tests positive for acetic acid. CONCLUSION • The unknown acid was determined to be acetic acid with an equivalence point pH of about 3.0 and a Ka approximately equal to that of acetic acid (see discussion). Our objective to identify the substance and reproduce the results was therefore met. • Future work includes determining the concentration of acetic acid in unlabeled bottles of vinegar by titration. EXPERIMENTAL • Sample Preparation: A KOH solution (~ 0.1 M) was prepared by adding • 4.105 g KOH to one L of water. 5.873 g of unknown solution was added • one liter of water under the assumption that acetic acid was the substance • and would yield a 0.01 molar solution. • Instrumentation: A titration funnel and stand were set up to titrate • the unknown. Phenylaline was used as a pH indicator. • Procedure: The unknown was titrated with KOH in order to find the • equivalence point volume. The same procedure was followed for acetic • acid3(see Figure 7-1,2also see • http://www.youtube.com/watch?v=g8jdCWC10vQ for video presentation). Works Cited 1. Sigmaaldrich.com. 2. Harris, D., Quantitative Chemical Analysis.2007, p. 122-127. 3. Titration Video. http://www.youtube.com/watch?v=g8jdCWC10vQ