Diprotic and Polyprotic Weak Acids

Introduction

The purpose of the experiment was to measure the pH of a solution to determine Ka for a polyprotic weak acid, use titration curves to calculate the molar mass of an unknown weak acid, and to determine the pH of a salt solution from analysis of a weak acid titration. A diprotic acid will be titrated with a known concentration solution of NaOH to determine the pKa, molar mass, and the identity of the acid. A diprotic acid contains two protons, which dissociates in water in two steps. Because there are two H+ atoms, the titration curve for a diprotic acid has two equivalence points that occurs where the moles of base are equal to the moles of acid in the solution. The reaction of the diprotic acid with NaOH are:

OH- + H2A (aq) → HA-(aq) + H2O(l). This is the reaction for the first equivalence point.

OH- + HA (aq) → A2-(aq) + H2O(l). This is the reaction through the second equivalence point.

Experimental Details

Before starting the experiment there are a couple safety warnings to consider. The experiment does call for the use of acidic and basic solutions, so it is important to handle them with care and if gets on your skin or clothing, rinse with water and NaHCO3 solution. The experiment required a standardized 0.10 M NaOH solution, an unknown weak acid, and an unknown salt. The experiment also called for the use of a LoggerPro software, which was calibrated using a known acid and base pH. Unknown B was used and 1.0026 grams were weighed and used to create an unknown acid. A known amount of 0.10 NaOH solution was poured into the buret, while about 25mL of the unknown acid was mixed with the unknown salt into 3 beakers. One beaker was placed under the buret and very small amounts of NaOH were dropped into the beaker until two curves appeared on the software creating a titration graph, and started to level out after the second curve.

Data and Calculations

Table 1: Masses

Unknown B 1.0026g

Acid pH = 4.01

Base pH = 10

Table 1 shows the mass...