Vernier Software & Technology

# 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 features the following Vernier sensors and equipment.

### Option 2

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

## Advanced Chemistry with Vernier

See other experiments from the lab book.

 1 The Determination of a Chemical Formula 2 The Determination of the Percent Water in a Compound 3 The Molar Mass of a Volatile Liquid 4 Using Freezing-Point Depression to Find Molecular Weight 5 The Molar Volume of a Gas 6 Standardizing a Solution of Sodium Hydroxide 7 Acid-Base Titration 8 An Oxidation-Reduction Titration: The Reaction of Fe2+ and Ce4+ 9 Determining the Mole Ratios in a Chemical Reaction 10 The Determination of an Equilibrium Constant 11 Investigating Indicators 12 The Decomposition of Hydrogen Peroxide 13 Determining the Enthalpy of a Chemical Reaction 14A Separation and Qualitative Analysis of Cations 14B Separation and Qualitative Analysis of Anions 15A The Synthesis of Alum 15B The Analysis of Alum 16 Conductimetric Titration and Gravimetric Determination of a Precipitate 17 Determining the Concentration of a Solution: Beer's Law 18 Liquid Chromatography 19 Buffers 20 Electrochemistry: Voltaic Cells 21 Electroplating 22 The Synthesis and Analysis of Aspirin 23 Determining the Ksp of Calcium Hydroxide 24 Determining Ka by the Half-Titration of a Weak Acid 25 The Rate and Order of a Chemical Reaction 26 The Enthalpy of Neutralization of Phosphoric Acid 27 α, β, and γ 28 Radiation Shielding 29 The Base Hydrolysis of Ethyl Acetate 30 Exploring the Properties of Gases 31 Determining Avogadro's Number 32 Potentiometric Titration of Hydrogen Peroxide 33 Determining the Half-Life of an Isotope 34 Vapor Pressure and Heat of Vaporization 35 Rate Determination and Activation Energy

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

#### Included in the Lab Book

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