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Objective: To isolate benzoic acid from the toluene-benzoic acid mixture by liquid-liquid extraction and purify the benzoic acid by recrystallization.

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´╗┐Experiment 1: Separation of organic compounds Experiment 2: Recrystallization and Determine melting point of organic compounds Objective: To isolate benzoic acid from the toluene-benzoic acid mixture by liquid-liquid extraction and purify the benzoic acid by recrystallization. Introduction Extraction is a powerful separation technique. Normally, it is useful when the compound of interest tend to be distributed itself in to one phase. In the separatory funnel, 2 phases are present , aqueous and organic phase. The distribution of each phases of a certain compound depends on its solubility and the solubility ratio is called the distribution coefficient, KD. Some approach can change the physical and chemical factors in order to obtain different distribution coefficient of a certain compound. For example, adding a large amount of sodium chloride to the aqueous solvent to produce a saturated solution. Crystallization is a method to purify solid organic compounds. First of all, selection of a suitable solvent for dissolving the solute of interest is required. Usually, the solute should be insoluble in the solvent at room temperature but we be soluble in the solvent at a higher temperature. Therefore, water is used in the experiment to dissolve benzoic acid. During the cool down process, the compound will crystallize but other impurities will be rejected from the crystal lattice and thus pure compound can be obtained. Melting points determination is able to find out the physical properties of a compound and also the purity. ...read more.


The conical flask was cooled down to room temperature and was then placed in an ice-bath. The recrystallized solid was filtered by suction filtration. The filter paper was placed on the Büchner funnel and was wet by de-ionized water. The conical flask was rinsed with small portion of de-ionized water. The crystals was dried for 10 minutes by the suction drawing air through the funnel and was then transferred onto a filter paper. Another filter paper was placed on the solid and two pieces of filter paper was pressed slightly for further drying. The crystals were weighed and was transferred into a clean and dry plastic b Result and Calculation: Experiment 1 (Extraction) Amount of toluene-benzoic acid mixture 30.0 mL Weight of product (1st conical flask) 2.354 g Weight of product (2nd conical flask) 0 g % Yield of theorem 60 g/L x 0.03 = 1.8 g % Yield of the extraction (2.354 g / 1.8 g) x 100% = 130.8% Table 1. The informations and data in experiment 1. Experiment 2 (Recrystallization) Amount of Raw Product used (Expt. 1) 2.21 g Weight of crystals (Expt. 2) 1.374 g % Yield of recrystallization 1.374 g / 2.21 g x 100% = 62.17% Color & Form of crystals Transparent & needle shape Efficiency of extraction and recrystallization of benzoic acid in toluene-benzoic acid mixture 1.374 g / 1.8 g x 100% = 76.3% Table 2. The informations and data in experiment 2. Melting Point Range (°C) Raw Product (Expt. ...read more.


Sodium benzoate was then converted to the benzoic acid by the reaction with 10% hydrochloric acid solution. There is no any observable benzoic acid in second conical flask which means the majority of benzoic acid was extracted in first conical flask. After extraction, the percentage yield of benzoic acid was 130.8%. The main reason may be water was not completely removed or other impurities were present. This reason can be further proved by the melting point of raw product. The melting point of raw product was 120-121 °C, which is lower than the benzoic acid standard. After crystallization, the percentage yield of crystals was 62.17%. Part of the benzoic acid may be lost during placing the conical on the steam bath because the temperature was too high and thus the benzoic acid was evaporated out. A narrow range of melting point of mixed product was obtained, which confirms the crystals are benzoic acid and relatively high purity of benzoic acid was obtained because of the same melting temperature to the standard. In order to obtain higher percentage yield of crystal products, at least 63.5mL of 10% aqueous hydrochloric is used instead of 35 mL. The temperature of steaming process should be lower to avoid evaporation of benzoic acid. Also, care should be taken to collect all the benzoic acid on the surface of Büchner funnel or on the filter paper. Conclusion: To conclude, the percentage yield of benzoic acid from the toluene-benzoic mixture was 76.3% and the melting point was 117.9-121°C. ...read more.

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