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# Determining Ka by the Half-Titration of a Weak Acid

## Introduction

A common analysis of a weak acid or a weak base is to conduct a titration with a base or acid of known molar concentration to help determine the equilibrium constant, Ka, for the weak acid or weak base. If this titration is conducted very carefully and very precisely, the results can lead to a valid approximation of an equilibrium constant. In this experiment, however, you will use a different technique to determine the Ka for a weak acid, acetic acid.

Your primary goal in this experiment is to calculate the Ka of acetic acid. The data that you will use to complete your calculations will come from the reaction of acetic acid with a solution of NaOH. Recall from your work with weak acid-strong base titrations that the point at which a reaction is half-titrated can be used to determine the pKa of the weak acid. In this experiment, the half-titration point will exist when you have added half as many moles of HC2H3O2 as moles of NaOH . Thus, OH will have reacted with half of the HC2H3O2, leaving the solution with equal moles of HC2H3O2 and C2H3O2. At this point, according to the Henderson-Hasselbalch equation,

$pH = p{K_a} + \log \frac{{[{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{3}}}{\text{O}}_{\text{2}}^ - ]}} {{[{\text{H}}{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{3}}}{{\text{O}}_{\text{2}}}]}}$

if there are equal moles of HC2H3O2 and C2H3O2 at the half-titration point, then pKa is equal to the pH value of the solution.

## Objectives

In this experiment, you will

• Conduct a reaction between solutions of a weak acid and sodium hydroxide.
• Determine the half-titration point of an acid-base reaction.
• Calculate the pKa and the Ka for the weak acid.

## Sensors and Equipment

This experiment requires each of the following Vernier sensors and equipment (unless otherwise noted):

You may also need an interface and software for data collection. What do I need for data collection?

The student-version preview includes:

• Step-by-step instructions for computer-based data collection
• List of materials and equipment

Note: The experiment preview of the computer edition does not include essential teacher information, safety tips, or sample data. Instructions for Logger Pro and other software (such as LabQuest App or TI handheld software, where available) are on the CD that accompanies the book. We strongly recommend that you purchase the book before performing experiments.

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### Experiment 24 from Advanced Chemistry with Vernier Lab Book

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#### Included in the Lab Book

Vernier lab books include a CD with word-processing files of the student instructions, essential teacher information, suggested answers, sample data and graphs, and more.