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Determining Ka by the Half-Titration of a Weak Acid

Figure from experiment 24 from Advanced Chemistry with Vernier

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 1

Option 2

Additional Requirements

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

Standards Correlations

See all standards correlations for Advanced Chemistry with Vernier »

Advanced Chemistry with Vernier

See other experiments from the lab book.

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

Experiment 24 from Advanced Chemistry with Vernier Lab Book

<i>Advanced Chemistry with Vernier</i> book cover

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