delivered and the base concentration. However, polyprotic acids, such as phosphoric acid in the cola drinks used in this experiment, have more than one dissociating proton per molecule. Each acidic proton is associated with its own Ka. For phosphoric acid, H3PO4, the balanced chemical equations for the standardization reaction and the titration reaction with each of its Kavalues are:KH C8H4O4(s)+NaOH(aq)→ NaK C8H4O4(aq)+H2O(l)−¿(aq)Ka1=7.5x10−3+¿(aq)+H2PO4¿H3PO4(aq)+H2O(l)↔H3O¿2−¿(aq)Ka2=6.2x10−8+¿(aq)+HPO4¿−¿(aq)+H2O(l)↔ H3O¿H2PO4¿3−¿(aq)Ka3=2.2x10−13+¿(aq)+PO4¿2−¿(aq)+H2O(l)↔ H3O¿HP O4¿My hypotheses concerning the results of this experiment is that because the values of Ka1,Ka2, and Ka3are separated from each other by several orders of magnitude, the dissociation may be regarded as taking place in a stepwise fashion, and therefore each successive Kais smaller than the previous. As a result, as the Kavalues become smaller, the pKavalues become bigger (pKa1= 2.1, pKa2= 7.2, and pKa3 = 12.7). The importance of the work undertaken in this experiment is that at the point halfway to an equivalence point, known as the halfway point, exactly half of the acid originally present has been neutralized by the base and as a result the
concentrations of initial acid and its conjugate will be equilibrium. At this point, Ka= [H3O+], and pKa= pH. The titration curve analysis provides a way to estimate Kavisually by reading the pH at halfway to any equivalence point. Data and ResultsCHEM 1212K LaboratoryExpt. 20: pH Titration of Phosphoric Acid in Cola DrinksA. Standardization of NaOHTable 1. Standardization of NaOH3 ptsTrialMass KHP (g)Volume NaOHSolution (mL)[NaOH] (mol/L)10.06431.70.01020.06530.60.010Mean [NaOH] (mol/L)0.010B. Titration of Cola SampleTable 2. Initial Parameters of TitrationVolume of Cola (mL)203 ptsInitial pH2.16Table 3. Data for Titration of Cola6 ptsVolume oftitrant (mL)pHpHV (ml)pH/VAvg. V oftitrant (mL)0.002.160.000.000.001.002.210.051.000.050.502.002.290.081.000.081.503.002.450.16
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- Fall '12
- pH, Sodium hydroxide, Phosphoric acid