272_Exp4BC

Only the oxalate ion remains c2o4 2 the concentration

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Unformatted text preview: affect the solution pH. At the equivalence point the first acidic proton has been removed and only the conjugate base remains (C2HO4- ). To determine the pH at this point, we have to remember that the conjugate base, C2HO4- , can now act as either an acid or a base. To determine which reaction predominates, we have to compare their equilibrium constants: C2 HO4 −( aq ) + H 2O( ) ⇔ C2O4 −2( aq ) + H 3O(+aq ) C2 HO4 − ( aq ) + H 2O( ) ⇔ C2 H 2O4 ( aq ) + OH −( aq ) K a = 6.1 × 10−5 K b = 1.54 × 10−13 The acid reaction will predominate, so we have to use an ICE table to determine the concentration of H+ and the pH of the solution. Before we can do that, we have to determine the concentration of the C2HO4- at the equivalence point: € − 0.0015 mol C2 HO4 = 0.038M (0.025L + 0.0157 L ) The number of moles of C2HO4- is determined from the initial volume and concentration of acid. The volume of the solution is the original volume (the 25 mL of oxalic acid) plus the volume of base added (15.7 mL), converted to liters for unit consistency. The ICE table, then, is: −2 − I C E C2 HO4 ⇔ C2 HO4 + H + 0.038 M 0 0 -x +x +x 0.038 - x x x Ka = € [C2O4−2 ][H + ] [C HO ] − 2 4 x2 0.038 − x x = [ H + ] = 0.0015 M 6.1 × 10−5 = pH = − log(0.0015 M ) = 2.82 Second equivalence point: If we continue titrating the sample with the strong base NaOH, then we will eventually reach a second equivalence point (there are two acidic protons on oxalic acid). Because the pKa of the second proton is small, the inflection point may not be as clear- cut as...
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