16 February 2004
Lab Report 1
The purpose of this project is to identify an unknown organic acid (X) by conducting various experiments to determine the acid’s unique properties. By determining selected constant properties of the unknown and then comparing these properties to the constant properties of known substances, it is possible to identify an unknown substance. The properties used to identify the unknown must be intensive, or independent of amount. Both chemical properties and physical properties are intensive properties used in determining the identity of an unknown (Maserton, p. 15). Because even small amounts of impurities systematically affect observed chemical reactions, the first step to determine the identity of an unknown is to purify the substance. One technique often used to purify solids is recrystallization from a selected solvent. Because of the variable solubility of different compounds in various solvents and at various temperatures, it is possible to remove impurities by dissolving the unknown and then immediately cooling the solvent. The unknown recrystallizes, but the impurities remain dissolved in the solvent. It is then a relatively simple matter to filter the pure crystals out of the impure solvent (Lab Manual, p. 32). Once the unknown has been purified, it is possible to begin determining its constant properties. One specific physical property that is helpful in identifying an unknown is the substance’s melting point (Lab Manual, p. 36). Equivalent weight, the weight in grams of the acid that contains exactly one mole acidic hydrogens, is also useful when identifying an unknown acid. Equivalent weight is determined by the pairing the unknown’s acidic hydrogens (H+) with known quantities of hydroxyl ions (OH-) through the process of titration. Before beginning a titration of the unknown acid, several practice titrations of HCl (hydrochloric acid) with a known concentration of NaOH (sodium hydroxide) were completed to gain expertise in this experimental technique (Lab Manual, p. 37).
MISSING FROM THIS INTRODUCTION IS THE DESCRITPTION OF THE UNKNOWNS
A sample of unknown acid (#XXX) with a mass of 0.1 g was placed in a clean test tube, and 1 ml of deionized H2O was added. This mixture was heated and stirred. The unknown did not dissolve, so 1 ml deionized H2O was again added to the mixture, and the heating and stirring was continued. The unknown had not dissolved after 3 ml deionized H2O had been added, so 1 ml ethanol (25%) was added to the mixture. Heating and stirring were continued. The unknown did not dissolve at 25%, 40%, or 50% ethanol. Another sample of unknown with a mass of 0.1 g was weighed out and 1 ml 100% ethanol was added. This mixture was heated and stirred. The unknown did not dissolve, so 1 ml ethanol was again added to the mixture. A sample of the unknown with a mass of 0.1 g dissolved in 2 ml pure ethanol. A proportion was set up and solved to determine that 160 ml of ethanol would dissolve 8.0 g of the unknown. 0.1 g X 8.0 g X
___________ = ____________ 2 ml ethanol r ml ethanol 0.1 r = 16
r = 160 ml
A sample of unknown weighing 8.000 g was placed in a beaker, and 160 ml ethanol was added. This mixture was heated and stirred until the unknown was completely dissolved. The container of dissolved unknown was placed into an ice bath and allowed to recrystallize. After the unknown was completely recrystallized, the crystals were vacuum filtered out of the solvent. The crystals were placed in a beaker with a mass of 107.359 g and covered with a watch glass. The beaker was set aside to allow the crystals to air dry completely. Some of the purified crystals remained in the solvent, so the solvent was poured into a flask and set aside to continue...