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+] = (Kax 0.09651)1/2 = (1.78x10-5x0.09651)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 of an unknown solid and be able to 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 • An unknown substance is suspected to be acetic acid, a chemical used in the manufacturing of household cleaning agents such as vinegar (also used in the food industry). Its chemical formula is H3C-COOH. • Gravimetric analysis is the study of the transfer of mass in a reaction. It is used in this experiment to compare the ratio of hydrogen ions to conjugate-base ions. This is done through converting mass to a unit that describes chemical ratios (1mole = molecular weight in grams). • Titration is a method of acid/base analysis, where a weak acid or base is treated with a strong base or acid respectively in order to determine the concentration of the weak species. • Acetic acid, suspected of being the unknown substance, reacts with a strong base in the following way: • AcOH+ OH-  AcO- + H2O • The acetate ion then undergoes the following equilibrium reaction: • AcO- + H2 O AcOH+ OH- • For every acid there is a constant of dissociation. If the constant for acetic acid is consistent with the experimental constant, then a match has been made. CONCLUSION EXPERIMENTAL • The unknown acid was determined to be Acetic Acid with an equivalence point pH of about 3.0 and a dissociation constant approximately equal to that of acetic acid. • Acetic acid is a chemical found in vinegar, used for pre-treating laundry , and cleaning floors and countertops. • Future work includes determining the concentration of Acetic acid in unlabeled bottles of vinegar by titration. • 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 an acetic identity 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. (see Figure 7-1)2 • Procedure: The unknown was titrated with KOH in order to find the • equivalence point volume. The same procedure was followed for acetic • acid.3 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