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ChemLab113-0056 - EXPERIMENT 5 Characterization of Aspirin...

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Unformatted text preview: EXPERIMENT 5 Characterization of Aspirin Introduction This week you will verify the identity and measure the purity of the aspirin you synthesized in . Experiment 4. Your measurements will be based on the fact that aspirin is a weak acid. The following molecular structure of aspirin was shown in the introduction to Experiment 4. 0‘0 O—E I é_CH3 —CH3 abbreviated: Q: i I ,OH COOH f? O The aspirin molecule contains a carboxyl (COOH) group. All organic compounds whose molecules contain carboxyl groups are weak acids of a class called carbogylic acids. The vast majority of organic compounds that function as acids are carboxylic acids. Salicylic acid and acetic acid are two other examples encountered in Experiment 4. The fact that aspirin is an acid means that you can analyze samples of it by titrating it with a base using the analytical skills you gained in Block I. You will use the base sodium hydroxide NaOI-I, which reacts with the hydrogen atom of the car-boxy] group of aspirin molecules as showu. R-COOH (aq) + NaOH (aq) —) R-COO' (aq) + H20 (1) + Na+ (aq) (The symbol R is commonly used to represent all portions of an organic molecule other than the group of interest, the carboxyl group in this case.) By measuring the volume of an NaOH solution of known molarity required to reach the equivalence point in the titration of a weighed aspirin sample, you can calculate a molar mass for the material. You should find a value near the 180.2 g/mol molar mass of aspirin, thus verifying the identity of the substance you synthesized. In addition to its molar mass we will use another property to characterize the aspirin pI‘OdUCt, namely its pKa. As we discussed in Experiment 2, when a weak acid is added to pure water it dissociates partially in reacting with water to produce hydroniurn ions. For example, the following equation shows the dissociation of aspirin. R—COOH (aq) + H20 (1) a R-COO‘ (as) + P130+ (3(4) The double arrows in the equation represent the fact that the dissociation is reversible and that a state of equilibrium is reached between the forward and reverse reactions. The equilibrium state for the dissociation of any carboxylic acid is characterized by the following equation. 79 ...
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