Diprotic Acids

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 counter calibration, pH calibration, and dilutions were employed. Determination of the weak acid yielded a molar mass of [pic] with a percent yield of[pic] and a pKa of [pic] with a percent yield of[pic]. The titration curve displayed a single equivalence point, indicating that Potassium Hydrogen Phthalate was the acid with a literature molar mass of [pic]and pKa of 5.432.

Introduction

The objective of this experiment was to determine the identity of an unknown weak acid through the characteristics of its corresponding titration curve such as the number of equivalence points, ¼, ½, ¾ equivalence points, and the volume of the first equivalence point. The weak acid-strong base titration curve gives insight into the unknown weak acid’s molar mass, Ka, and protonation.

For the purposes of this titration, the standardization of the NaOH solution was required for precision and accuracy. Through manual titration of a known amount of KHP, the concentration of the NaOH base was effectively determined.

The assumption that the moles of acid equal the moles of base is derived from the sensitivity of the equivalence point. In the case of a titration, the equivalence point determines the molar mass of the acid through the assumption that at that point, the moles of acid added...