Use of Gravimetric Analysis in Determining Mass Percent of Unknown Chloride - Lab Report Example

Chemistry 116AL – Unknown Chloride Lab Report TA Unknown Sample Number: Use of Gravimetric Analysis in Determining Mass Percent of Unknown Chloride Abstract Subsequent sections of this report contain detailed procedures used in determining the mass percent of chloride in an unknown sample. First, the introduction part of the paper covers intensively on the theoretical background of quantitative analysis, precisely on the use of gravimetric analysis in physical chemistry. Principles of gravimetric analysis are sufficiently covered, including the conditions that must be ascertained before employing the precipitation technique of gravimetric analysis. Subsequently, the practical applicability of the analysis method was illustrated within the experimental procedures section. Resultant data of each procedural step were tabulated for easy interpretation. In addition, the report contains a thorough discussion of the results, and finally a conclusive assertion of the experiment’s effectiveness as a quantitative analysis exercise. Introduction In physical chemistry, one method used in the quantitative determination of unknown substances is gravimetric analysis. Technically, gravimetric analysis falls into two categories; precipitation method, and volatilization method. In precipitation method, an unknown substance is determined by isolating its ion in a solution through precipitation (Smith and Stanton, 111). After isolation through precipitation, other subsequent steps in this method of analysis include filtering, conversion of the precipitate to a known compound, and determination of the compound’s mass through weighing. Primarily, the principle behind gravimetric analysis entails precisely determining the mass of an element of interest in a pure precipitated compound, followed by using the determined mass to derive the mass percentage of the element in an unknown compound. Prior to employing gravimetric analysis in chemistry, certain essential conditions must be ascertained. First, the ionic form of an element under study should be completely precipitated. Second, the precipitated compound containing the element must be pure, and finally, the resultant precipitate should be easily filtered from the mother liquor. This lab report covers on the employment of gravimetric analysis in the mass determination of chlorine within an unknown chloride solution. In determining the mass percent of chlorine, a pure precipitate containing chloride ions must be prepared. Among the elements that form insoluble chlorides include Pb2+, Hg22+ and Ag+ (Smith and Stanton, 114). This experiment used silver because it is not only easily filtered and readily available, but also undergoes 99.99% conversion from Ag to silver chloride. Chloride ions are made by dissolving chlorine in nitric acid. On the other hand, silver ions are contained within a solution of silver nitrate. Ionic equation for the reaction is written as; Ag+ (aq) + Cl- (aq) → AgCl(s) In order to improve the percentage conversion and formation of AgCl, appropriate reaction conditions are maintained, which include but not limited to use of excess silver nitrate, and sustenance of temperatures at approximately 95oC. After precipitation of AgCl(s), sufficient washing and filtration procedures are employed to increase the precipitate’s purity before the pure compound is dried and weighed. Experimental Procedures First, four filter crucibles were cleaned by rinsing with a mixture of 15 ml of deionized water and 6M nitric acid. Subsequently, the four crucibles were dried at 120oC for approximately 2 hours, and their individual mass recorded to 0.1 mg. The unknown samples were first dried at 120oC for 2 approximately 2 hours and the cooled in desiccators before weighing 0.4 grams of the sample into each dried crucible (Smith and Stanton, 114). Subsequently, each weighed sample was quantitatively transferred into a 400 ml beaker, and the sample in the beaker dissolved with a mixture of 200 ml water and 5 ml of 6 M nitric acid. Upon complete dissolution of the sample, 0.2 M silver nitrate was added into the beakers and stirred until a precipitate began to coagulate. While the temperature was maintained at 95oC for 10 minutes, excess silver nitrate was added into the beaker and continually stirred until formation of the precipitate ceased. The resultant precipitate was carefully decanted into one of the weighed filter crucibles and dried at 110oC for 2 hours. Eventually, the dry precipitate was cooled, weighed, and its mass recorded in the data table (Smith and Stanton, 114). As a means of enhancing precision, four independent trials of the entire process were repeated, and the mass of each pure precipitate was recorded in the data table below. Data and Results Unknown Data Trial Sample mass (g) AgCl mass (g) Mass % of Cl- 1 0.4033 0.8756 53.54 2 0.4087 0.8766 52.89 3 0.4176 0.8865 52.35 4 0.4093 0.8980 54.10 Mean mass % of Cl- 53.22 95% confidence interval 1.21 Discussion In determining the mass percent of chloride in the unknown sample, the following calculations were preformed. The first trial of the experiment weighed 0.44033 g of the unknown chloride sample. After dissolution of the sample with nitric acid followed by addition of the silver nitrate to the solution, 0.8756 g of silver chloride was obtained. Based on the ionic equation Ag+ (aq) + Cl- (aq) → AgCl(s), it is acknowledgeable that one mole of Ag+ combined with one mole of Cl- to form one mole of AgCl. Therefore, (0.8756 g of AgCl) / (143.323 g/mol; molar mass of AgCl) = 0.006109 moles of AgCl. Correspondingly, moles of Cl- that reacted = 0.006109. Mass of chloride that reacted was given by 0.006109 × 35.453 = 0.2166 g (Smith and Stanton, 116). Therefore, the mass percent of chlorine in the unknown sample was given by (0.2166 g Cl) / (0.4033 g of sample) × 100 = 53.54%. The percentage mass of each trial was calculated using the procedure illustrated above. In average, a mean mass percent of 53.22 was obtained. In order to factor in the effects of errors encountered during the experiment, the final result was reported at 95% confidence level. Consequently, the reliable mass percent of chloride in the unknown sample is 53.22% +/- 1.21. This means that the actual mass percent of the chloride ranges from 52.01% to 54.43%. Evidently, an interval of 1.21 at 95% confidence limit widens the precision range of the chloride’s mass percent. Supposedly, this large confidence interval resulted from multiple errors that infiltrated into the experiment’s data. Among the possible errors include outright blunders, and observational errors (Smith and Stanton, 118). With respect to outright blunders, each trial in the experiment involved obtaining repeated readings to 4 decimal places. In this regard, there is a remote yet distinct possibility that some values were either omitted or added during the recording process. With respect to observational errors, the element of parallax in reading the meter scales may have caused significant inconsistencies. Admittedly, prevalence of such errors throughout the experiment is responsible for a substantial portion of the wide confidence interval. Conclusion At this juncture, it emerges that gravimetric analysis is instrumental in the determination of quantitative parameters in physical chemistry. The precipitation technique is particularly important in quantitative analysis of individual elements within insoluble salts like silver chloride. Apparently, determination of chloride in the unknown sample was made easy by the aforementioned conditions of gravimetric analysis. In order to minimize the effect of errors and enhance reliability of findings, each procedure must be performed with due caution (Smith and Stanton, 118). Otherwise, reporting results within confidence intervals plays a significant role in mitigating the effects of experimental errors. In conclusion, it is undeniable that the precipitation technique of gravimetric analysis proved effective in yielding the experiment’s objective results. Work Cited Smith, Charles and Stanton, Bobby. Experiments in General Chemistry: Gravimetric Analysis of a Chloride, Sulfate or Carbonate Compound. Pittsburg: Cengage Learning, 2009. Print. Read More