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Diprotic Acids Essay

2207 words - 9 pages

BB6
11/30/07
Characterization of a Weak Acid
Abstract
Weak acids are characterized by their equilibrium Ka, molar mass, and protonation. These characteristics were determined through a weak acid-strong base titration curve using standardized NaOH. The number of equivalence points determined the protonation-whether it was monoprotic or diprotic. The first equivalence point described the molar mass of the unknown. The Ka was determined through the ¼, ½, ¾ equivalence points using the Henderson-Hasselbach Equation. Laboratory techniques such as weighing by difference, use of indicator, manual titrations, Vernier Lab Pro interface, automatic titrations, volumetric pipetting, volume-drop ...view middle of the document...

Since the acid was the dominant source of H+ ions and OH- is an extremely strong base, the titration reaction HA(aq) + OH-(aq) ( H2O(l ) + A-(aq) occurs to near completion.
The reaction suggests that one mole of OH- ions react with one mole of HA molecules. This is essentially valid since the HA is the major proton donor. If the acid and base react in a one to one ratio, the equivalence point indicates where the moles of acid are equal to the moles of base in solution. The determination of the molecular weight simply requires the grams of acid added over the moles of acid.
Monoprotic acids remain in a state of equilibrium described by HA(aq) ⇌ H+(aq) + A-(aq) with [pic]. Like the Keq of a general reaction, the Ka posses similar properties such as temperature dependence. However, the auto ionization of water is excluded from the acid reaction because it remains at a standard concentration. The derivation of the Henderson-Hasselbach equation comes from the log of the Ka equilibrium equation. Thus, Ka, pH, and the ratio of acid to base are described by[pic]. Using the ½ equivalence point where the term [pic] equals 0, the pKa is determined by the pH. The pKa could also be calculated at varying points provided that the ratio of [pic]is known.
In the case of a diprotic acid, two equations and two equilibrium points such as H2A(aq) + OH-(aq) ( H2O(l ) + HA-(aq) with [pic] and HA-(aq) + OH-(aq) ( H2O(l ) + A2-(aq) with [pic] would be required for the characterization. However, the determination of diprotic acids is roughly similar to the determination of a monoprotic acid.
Acid-base titration curves are not limited in the determination of unknown weak acids. Titration curves were also required to determine the purity of single walled carbon nanotubules. Simple acid base titrations were conducted to determine the amount of carboxylic acid groups present in carbon nanotubles, a measurement of its purity. Acid base titrations provide an accurate and inexpensive means for testing the purity of these structurally important molecules (1).

Experimental
• The procedure of this experiment was conducted following the procedure of “An Introduction to Chemical Systems in the Laboratory” pages 50-53 (2).
• Instead of creating a 0.1M NaOH solution, 5.1 mL of 19.1M NaOH was diluted in 1L of water to form 0.094M NaOH solution
• Instead of weighing 0.3g of KHP, 0.3076g, 0.3145g, 0.3058g of KHP was weighed.
• Three drops of phenolphthalein were added to each trial
• Instead of automatic volume calibration, manual calibrations were made using number of drops and volume.
• Instead of weighing 0.30g of unknown acid, 0.3068g, 0.3038g, 0.3033g was weighed. The second trial was weighed twice because of a failed titration, initially 0.2994g were weighed.
• Acids were diluted in 75ml of water precisely using a 25ml volumetric pipette
• Titration was stopped at a pH of12.51, 12.50, 12.01 in a range between...

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