| 1. | Safety |
| 2. |
Objectives/Overview |
| 3. |
Procedures |
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4. |
Observations |
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| 8. | Conclusions |
| 9. | Calculations/Set-Ups |
| 10. | Grading Scale |
| 11. | Review Prelab Questions |
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12. |
Review Postlab Questions |
Experiment
17
Titration
of a Diprotic Acid: Identifying an Unknown
OBJECTIVES
A di-protic acid is an acid that yields two H+ ions per acid molecule. Two examples of di-protic acids are sulfuric acid, H2SO4, and carbonic acid, H2CO3. A di-protic acid dissociates in water in two stages:
Because of the successive dissociation, titration curves of di-protic acids have two equivalence points, as shown in Figure 1. The equations for the acid-base reactions occurring between a di-protic acid, H2X, and sodium hydroxide, NaOH, are:
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from the beginning to the first equivalence point:
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Figure
1  |
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| from the frist to the second equivalence point:
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from the beginning through the second equivalence point (net reaction):
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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 di-protic acid by finding its molar mass. A di-protic acid is titrated with NaOH solution of known concentration. Molecular weight (or molar mass) is found in g/mole of the di-protic 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). Note that it takes two moles of NaOH to reach the second equivalence point. Once grams and moles of the di-protic 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 will use the second equivalence point to calculate molecular weight, in this experiment, because the second equivalence point is more clearly defined on the titration curve. Simply divide the NaOH volume used to reach the second point by 2 to confirm the first equivalence point; or from Equation 5, use the ratio: 1 mole H2X / 2 mole NaOH
