Titration of a Diprotic Acid PDF

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26th April 2017

Caitlin Bettenay

Titration of a Diprotic Acid: Identifying an Unknown

ABSTRACT: The purpose of the experiment was to identify an unknown diprotic acid by finding its molecular weight. This was done through the process of titration of the diprotic acid with NaOH solution. The calculated molecular weight was 109.09 g/mol. In comparing this value to other diprotic acid’s molecular weights, the unknown diprotic acid was determined to be two possible solutions: the unknown acid is either Malonic acid (molecular weight = 104 g/mol) or Maleic acid (molecular weight = 116 g/mol). Though using additional calculations, it was determined that the unknown diprotic acid was Maleic acid. The percent error for the Maleic acid for the 1st equivalence point was 5.96% while for the second equivalence point it was 5.7%. INTRODUCTION: The purpose of the experiment is to identify an unknown diprotic acid by finding its molecular weight using titration. An acid/base titration is performed by carefully adding one solution from a buret, the titrant, to another substance in a beaker until all of the substance in the flask has reacted. A diprotic acid is a class of Arrenius acids that dissociates in two stages and yields two H+ ions per acid molecule: (1) H2X(aq)

   H+(aq) + HX-(aq)

(2) HX-(aq)

   H+(aq) + X2-(aq)

Because of the successive dissociations, the titration curves of diprotic acids have two equivalence points. The equations for the acid-base reactions occurring between a diprotic acid, H2X, and NaOH, are:

from the beginning to the 1st equivalence point: (3) H2X + NaOH

   NaHX + H2O 1

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Caitlin Bettenay

from the 1st to the 2nd equivalence point: (4) NaHX + NaOH

  Na2X + H2O

from the beginning of the reaction through the 2nd equivalence point (net reaction): (5) H2X + 2NaOH

  Na2X + 2H2O

The titration curve shown below is a general graph of the dissociation of a diprotic acid. In the process, there are two buffering regions and two equivalence points.

Figure 1: Titration Curve of a Strong Base Titrating a Polyprotic Acid

In this experiment, an unknown diprotic acid is titrated with a NaOH solution of known concentration. Weighing the given sample of acid will reveal its mass in grams, and the number of moles of the acid can be determined from the volume of NaOH titrant needed to reach the 1st equivalence point, as by definition the equivalence point is when chemical equilibrium, in which the moles of the acid and the moles of the base are the same, is reached. The volume and the concentration of NaOH titrant are used to calculate moles of NaOH and the molecular weight will be calculated. Either the 1st or 2nd equivalence point can be used to calculate the molecular weight of the diprotic acid.

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Data:

Figure 2: Graph Displaying the Titration Curve of the Unknown Diprotic Acid.

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Figure 3: Graph displaying the First Derivative Plot of Titration Curve

Figure 4: Graph Displaying the Second Derivative Plot of the Titration Curve

Table 1: Table of calculated results and raw data including; mass of diprotic acid (g), Concentration of NaOH for the equivalence points (M), pH at equivalence points, volume at equivalence points (mL), NaOH volume before and after equivalence pt. (mL), moles diprotic acid at equivalence points (mols), moles NaOH at equivalence points (mols), molecular weight of calculated diprotic acid at each equivalence point (g), other diprotic acid information, % error and Ka values. Mass of diprotic acid (g) Concentration of NaOH (M)

0.120g 0.1 M

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Caitlin Bettenay Equivalence pt. pH:

Volume at 1 st equivalence pt. (mL) Volume at 2nd equivalence pt. (mL) NaOH Volume before equivalence pt. (mL) NaOH Volume after equivalence pt. (mL) Moles of diprotic acid 1st equivalence pt: Moles of diprotic acid 2st equivalence pt: Moles of NaOH

Molecular weight of calculated diprotic acid 1st equivalence pt. Molecular weight of calculated diprotic acid 2nd equivalence pt. Name, formula, and accepted molecular weight of the diprotic acid/s for 1st equivalence pt. Name, formula, and accepted molecular weight of the diprotic acid/s for 2nd equivalence pt. % error for Malonic acid 1st equivalence pt. % error Maleic acid 1st equivalence pt. % error Maleic acid 2nd equivalence pt. % error Malic acid 2nd equivalence pt. Ka1 (first equivalence point) Ka2 (second equivalence point)

pH 1st ½ equivalence pt: 2.80 pH 1st equivalence pt: 3.70 pH 2nd ½ equivalence point: 6.25 pH 2nd equivalence point: 8.99 1st volume 9.8 mL nd 2 volume 21.71 mL 21.61mL 21.82mL

0.0011 mols −4 mols 9.79 ×10 1st equivalence pt. −4 mols 9.79 ×10 nd 2 equivalence pt. 2.1715 10−3 mols 109.09 g/mol

122.6 g/mol

Malonic Acid, H2C3H2O4, 104g Maleic acid, H2C4H2O4, 116 g Maleic acid, H2C4H2O4, 116 g Malic acid, H2C4H4O5, 134g 4.89% 5.96%

5.7% 8.5%

0.00158 −7

5.62 × 10

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Discussion: The purpose of the experiment was to identify an unknown diprotic acid by finding its molecular weight. This was done through the process of titration of the diprotic acid with NaOH solution. At the first equivalence point, all H+ ions from the first dissociation reacted with the NaOH base. At the second equivalence point, all H+ ions from both reactions have reacted (twice as many as at the first equivalence point). Therefore, the volume of NaOH added at the second equivalence point is approximately twice that of the first equivalence point. As can be seen from the information provided in Table 1 and Figure 2, its clear that the second equivalence point was more clearly defined as it had larger changes in pH and a steeper slope.

When polyprotic acids are titrated with strong bases, there are multiple equivalence points because polyprotic acids lose their protons in a stepwise manner. At either equivalence point, there were approximately twice as many mols of NaOH base for every mol of diprotic acid because there is 2:1 stoichiometric ratio between the two species of the reaction. This fundamental mathematical relationship between NaOH and the diprotic acid used in the titration allowed for the subsequent calculation of the grams of the unknown diprotic acid and revealed its identity.

The unknown diprotic acid used in the titration was found to be maleic acid, H2C4H2O4, 116g, based on the percent error and the equivalence point calculations as can be seen in Table 1. The experimentally determined molecular weight of the diprotic acid from the 2nd equivalence point was 122.6mol, which has a 5.7% error from the accepted molecular weight of 116g/mol. While the results gathered are consistent and sound, there were a few potential sources of error. One of these was that there was a lag time between the addition of the NaOH to the diprotic acid solution and the reading of the pH sensor in the beaker containing the mixture. This could have resulted in inaccurate readings between the measurement and when the solution was mixed by the magnetic stir bar. This would result in deviations of the accepted value if the calculated or observed value was

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used in more than one step in the calculated equations. In future testing, it would seem appropriate to do more titrations in order to gain more results so that the calculations can be more accurate, reliable and valid. Although there were errors, the trend lines of the data remained unaffected, thus, the results gathered can be said to be reliable.

Question: When the pH of a solution equals the pKa, this is known as the + equivalence point of that solution. When all the H ions are used up by the OH ions. Thus, the pKa of this equation in the first equivalence point would be 4.12, while the pKa of the second equivalence point would be 8.84.

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