Recrystallization of Benzoic Lab Report (Lab Report Sample)

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Recrystallization of Benzoic Lab Report
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Introduction
The objective of this experiment is to isolate benzoic acid from an impure sample by use of a recrystallization. Another objective is to learn recrystallization technique. Percentage recovery of benzoic acid will also be determined by recrystallization method (One Part of Chemistry 2011).
         A pure substance is a homogeneous substance which contains of molecules which have the same structure and molecular weight. However, some substances which are believed to be pure can possibly contain tiny quantities of contaminants. This involves synthesis of products during the reaction, unused reactants, inorganic substances, and solvents. Recrystallization method may be utilized in order to purify a sample and thus help get rid of the impurities (One Part of Chemistry 2011).
Recrystallization utilizes the advantage on the differences in terms of the solubility of the products of interest and impurities to acquire the purified desired products. Nearly all solutes are soluble when placed in hot solvents than when in cold solvents. Consequently, when a sample dissolves in a hot solvent although it is inadequate to dissolve it by use of a cold solvent, the crystals are expected to form as the hot solution is left to cool. In some cases impurities which are present in a given sample are more insoluble with the better choice of the best solvent. What will remain after afterwards is mostly the sample of interest (One Part of Chemistry 2011).
Apparatus and Materials
Erlenmeyer flask (125 cm3), a funnel, a hotplate, beaker, boiling chips, charcoal and benzoic acid
Procedure
1.0 g of a sample of crude benzoic acid was weighed and placed into a 125 cm3 Erlenmeyer flask. Then 100 cm3 of water was boiled in a beaker which appropriately placed on a hotplate together boiling chips. An Erlenmeyer flask which contained little water and boiling chips was then heated on a hotplate, with a funnel positioned on its neck.
A filter paper was the consequently fluted so as to fit the funnel. Some boiling chips were then added into benzoic acid. Hot water was then added drop by drop into the benzoic acid until it begun to dissolve. At this stage 0.2 g of activated charcoal was then applied in order to decolorize it. The hot solution was then filtered via fluted filter paper. The filtrate was collected in a heated flask (One Part of Chemistry 2011).
The originally used flask with its filter paper was then rinsed by use of a little hot water. The benzoic acid solution was then taken off the hot plate and afterwards allowed to cool at the room temperature and pressure conditions for 5 minutes. Benzoic acid solution was then cooled in an ice bath for about 10 minutes. Benzoic acid crystals were then harvested by use of a suction filtration method. Vacuum was allowed to pull on the funnel for about 5 minutes. The filter paper containing crystals was then transferred on to a clean piece of filter paper. The crystals were then allowed to air-dry. The percentage recoveries as well the melting point of benzoic acid were determined (One Part of Chemistry 2011).
Results and Discussion
Weight of crude benzoic acid was found to be 2.0007 g and that of the filter paper was 0.8459 g. Weight of benzoic acid crystals together with the filter paper was 1.8776 g and that of benzoic acid crystals was measured as 1.0317 g. The melting point of the collected benzoic acid crystals was determined as 120 °C. Percent Recovery is defined as the ratio weight of the compound recovered against the weight of the original sample and then expressed as a percentage. Percent recovery was therefore determined as: (1.0317 ÷ 2.0007) * 100 = 51.57%. Loss of product is therefore 48.87%. This value of loss was much higher might be due addition of water. Thus the product must have been lost to the solution. In addition some of the product must have been adsorbed on the surfaces of the activated charcoal (One Part of Chemistry 2011).
Relative Accuracy Melting Point of the experiment was determined by the calculation of the ratio melting point of benzoic acid to that of recovered benzoic acid by 100. Thus Relative Accuracy of Melting Point was 120/122 * 100 = 98.36% (One Part of Chemistry 2011).
Conclusion
Excess use of activated charcoal leads to collection of grey crystals. Right quantities of charcoal should therefore be determined for one to harvest quality and pure crystals by the process of recrystallization. Only a small part of charcoal is required for the decolorizing effect. The percent yield was found to be 51.57% may be due to the loss to solution by over dilution effect as well as charcoal adsorption. More improvements are then highly recommendable in order to better the results (One Part of Chemistry 2011).
Answers to the Questions on the End the Laboratory Manual
1 During recrystallization, an orange solution of a compound in hot alcohol was treated with activated carbon and then filtered through fluted paper on cooling; the filtrate gave grey crystals, although the compound was reported as colorless. Explain why the crystals were gray and describe steps that you would take to obtain a colorless product.
The collected crystals should have been contaminated with charcoal powder. More than necessary activated charcoal must have been used in the process. To correct for this outcome, only 1 to 5% of the carbon bys mass will be used as compared to the original mass of the sample which is being purified. Only small quantities of charcoal are enough for the purposes of decolorization.
2 The solubility of compound A is ethanol is 0.8g/ 100mL at 0 °C and 5.0g per 100mL at 78°C. What is the minimum amount of ethanol needed to recrystallize 12.0g sample of compound A? How much would be lost in recrystallization, that is, would remain in the cold solvent?
Let R to be the volume of ethanol required for recrystallization and S be the m...