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Titration of a Diprotic Acid: Identifying an Unknown

Figure from experiment 25 from Chemistry with Vernier

Introduction

A diprotic acid is an acid that yields two H+ ions per acid molecule. Examples of diprotic acids are sulfuric acid, H2SO4, and carbonic acid, H2CO3. A diprotic acid dissociates in water in two stages:

{\text{(1)  }}{{\text{H}}_{\text{2}}}{\text{X(aq)}} \leftrightarrow {{\text{H}}^{\text{ + }}}{\text{(aq)  +  H}}{{\text{X}}^{\text{ - }}}{\text{(aq)}} {\text{(2)  H}}{{\text{X}}^{\text{ - }}}{\text{(aq)}} \leftrightarrow {{\text{H}}^{\text{ + }}}{\text{(aq)  +  }}{{\text{X}}^{{\text{2 - }}}}{\text{(aq)}}

Because of the successive dissociations, titration curves of diprotic acids have two equivalence points, as shown in Figure 1. The equations for the acid-base reactions occurring between a diprotic acid, H2X, and sodium hydroxide base, NaOH, are from the beginning to the first equivalence point:

{\text{(3)  }}{{\text{H}}_{\text{2}}}{\text{X  +  NaOH}} \leftrightarrow {\text{NaHX  +  }}{{\text{H}}_{\text{2}}}{\text{O}}

from the first to the second equivalence point:

{\text{(4)  NaHX  +  NaOH}} \leftrightarrow {\text{N}}{{\text{a}}_{\text{2}}}{\text{X  +  }}{{\text{H}}_{\text{2}}}{\text{O}}

from the beginning of the reaction through the second equivalence point (net reaction):

{\text{(5)  }}{{\text{H}}_{\text{2}}}{\text{X  +  2 NaOH}} \leftrightarrow {\text{N}}{{\text{a}}_{\text{2}}}{\text{X  +  2 }}{{\text{H}}_{\text{2}}}{\text{O}}

At the first equivalence point, all H+ ions from the first dissociation have reacted with NaOH base. At the second equivalence point, all H+ ions from both reactions have reacted (twice as many as at the first equivalence point). Therefore, the volume of NaOH added at the second equivalence point is exactly twice that of the first equivalence point (see Equations 3 and 5).

The primary purpose of this experiment is to identify an unknown diprotic acid by finding its molecular weight. A diprotic acid is titrated with NaOH solution of known concentration. Molecular weight (or molar mass) is found in g/mole of the diprotic acid. Weighing the original sample of acid will tell you its mass in grams. Moles can be determined from the volume of NaOH titrant needed to reach the first equivalence point. The volume and the concentration of NaOH titrant are used to calculate moles of NaOH. Moles of unknown acid equal moles of NaOH at the first equivalence point (see Equation 3). Once grams and moles of the diprotic acid are known, molecular weight can be calculated, in g/mole. Molecular weight determination is a common way of identifying an unknown substance in chemistry.

You may use either the first or second equivalence point to calculate molecular weight. The first is somewhat easier, because moles of NaOH are equal to moles of H2X (see Equation 3). If the second equivalence point is more clearly defined on the titration curve, however, simply divide its NaOH volume by 2 to confirm the first equivalence point; or from Equation 5, use the ratio:

{\text{1 mol }}{{\text{H}}_{\text{2}}}{\text{X / 2 mol NaOH}}

Objectives

In this experiment, you will identify an unknown diprotic acid by finding its molecular weight.

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 Chemistry with Vernier »

Chemistry with Vernier

See other experiments from the lab book.

1Endothermic and Exothermic Reactions
2Freezing and Melting of Water
3Another Look at Freezing Temperature
4Heat of Fusion of Ice
5Find the Relationship: An Exercise in Graphing Analysis
6Boyle's Law: Pressure-Volume Relationship in Gases
7Pressure-Temperature Relationship in Gases
8Fractional Distillation
9Evaporation and Intermolecular Attractions
10Vapor Pressure of Liquids
11Determining the Concentration of a Solution: Beer's Law
12Effect of Temperature on Solubility of a Salt
13Properties of Solutions: Electrolytes and Non-Electrolytes
14Conductivity of Solutions: The Effect of Concentration
15Using Freezing Point Depression to Find Molecular Weight
16Energy Content of Foods
17Energy Content of Fuels
18Additivity of Heats of Reaction: Hess's Law
19Heat of Combustion: Magnesium
20Chemical Equilibrium: Finding a Constant, Kc
21Household Acids and Bases
22Acid Rain
23Titration Curves of Strong and Weak Acids and Bases
24Acid-Base Titration
25Titration of a Diprotic Acid: Identifying an Unknown
26Using Conductivity to Find an Equivalence Point
27Acid Dissociation Constant, Ka
28Establishing a Table of Reduction Potentials: Micro-Voltaic Cells
29Lead Storage Batteries
30Rate Law Determination of the Crystal Violet Reaction
31Time-Release Vitamin C Tablets
32The Buffer in Lemonade
33Determining the Free Chlorine Content of Swimming Pool Water
34Determining the Quantity of Iron in a Vitamin Tablet
35Determining the Phosphoric Acid Content in Soft Drinks
36Microscale Acid-Base Titration

Experiment 25 from Chemistry with Vernier Lab Book

<em>Chemistry with Vernier</em> book cover

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