The purpose of the experiment is to determine the percent composition of calcium in unknown salt through the gravimetric analysis. Essentially, the approach of gravimetric analysis allows the measurement of the mass of a compound using partial information. Since salts consist of metal and non-metal components, it is possible to establish one part of it based on stoichiometric proportions of equations or empirical formula of a compound. In this experiment, this form of the gravimetric analysis entails precipitation of calcium in the salt and estimation of the amount of mass formed in the reaction. Starting from unknown salt with the identified component of ionic calcium, reaction with oxalate anion produces a precipitate of a hydrated salt of calcium oxalate, which is easy to measure and determine its mass based on a net ionic equation.
The net ionic equation: Ca2+ (aq) + C2O22- (aq) + H2O (l) ???? CaC2O4. H2O (s)
The net ion equation shows that calcium cation and oxalate anions react with a mole ratio of 1:1. The precipitative gravimetric analysis allows the use of a net ionic equation for the measurement of the mass of solid calcium oxalate, determination of percent composition of calcium in the salt using the empirical formula, and calculation of the mass of calcium in unknown starting salt. The assumption is that the amount of calcium precipitated is the same as the mass of calcium in the unknown salt. The unknown calcium salt (CaXy) comprises a specific proportion of calcium and compound X with definite empirical formula Xy. The advantages of using gravimetric analysis in determining the mass of calcium in unknown salt are it offers accurate measurements and does not use sophisticated apparatus in the laboratory.
Procedure
Gravimetric analysis of calcium compound started with the assembly of the required apparatus and reagents. As apparatus, lab coats, gloves, eyes goggles, two beakers, flasks, funnel, filter paper, droppers, watch glass, graduated cylinders, Buchner apparatus, heater, and weighing balance were assembled for the experiment. Calcium carbonate salt (Ca2CO3), hydrochloric acid, oxalate, methyl-red indicator, deionized water, and acetone were gathered as reagents required in the experiment. Protective clothing was put on before the experiment started to ensure the safety of handling chemicals and apparatus in the laboratory.
Enough stock solution for the experiment was prepared to allow sharing with other partners. The stock solution was prepared by diluting the unknown salt in acidified deionized water. Using a measuring cylinder, 96.2 ml of deionized water was measured and placed in a 250 ml Erlenmeyer flask. In addition, 1.239 g mass of the unknown solid was weighed in a balance, mass recorded and dissolved in deionized water. Next, 3.8 ml of 12 M hydrochloric acids was measured using a 10 ml graduated measuring cylinder. The measured acid was added into the flask in the fume chamber and stirred gently to make 100 ml of stock solution to be shared by partners in the experiment.
A 50 ml graduated measuring cylinder was rinsed with portions of stock solution several times and disposing them in a waste beaker before using it to measure the required volume of stock solution. Out of 100 ml of the stock solution prepared, 25 ml was measured using a cleaned 50 ml measuring cylinder and placed into a separate 250 ml beaker. Another 25 ml of the stock solution was measured and put into another separate 250 ml beaker. Each solution in the beakers was acidified by adding 75 ml of 0.1M hydrochloric acid and mixing them gently. To enable the determination of the endpoint, five drops of methyl-red were added into each solution in two beakers.
After the acidification of the stock solutions in beakers, reactants were set to allow the formation of a precipitate of calcium oxalate. To precipitate the solutions in beakers, 25 ml of 0.3M ammonium oxalate was added into the beakers while stirring with a glass rod to mix. Next, 15 g of solid urea was weighed, placed into the beakers with acidified stock solution, and heated to boil for 40 minutes on a hot plate. During heating, urea decomposed into carbon dioxide and ammonia. Subsequently, produced ammonia reacted with oxalic acid by causing its deprotonation into oxalate and itself protonated into ammonium ion. Calcium ions in the solution reacted with the produced oxalate to generate calcium oxalate hydrate as the desired precipitate. While waiting for the solutions to boil for 40 minutes and reaction to occur, watch class and filter paper were pre-weighed and recorded before being used to measure the amount of precipitate formed. The endpoint of precipitate was determined by observing the color change from red to yellow-orange. The Buchner apparatus powered by running water was used to filter and dry precipitate via vacuum aspiration. About 7 ml of acetone was used to wash and dry the precipitate twice. The mass of precipitate formed was weighed in a watch glass and filter paper and results were recorded.
Preliminary calculations of the expected composition of elements in calcium oxalate (CaC2O4.H2O) were performed as tabulated below. Based on the empirical formula of calcium oxalate monohydrate, the atomic masses of each element were identified and multiplied by the number of respective atoms to obtain the molecular mass.
Table 1: Calculations of molecular mass of calcium oxalate monohydrate (CaC2O4.H2O)
The summation of each of the total atomic masses gives the molecular mass of the hydrated calcium oxalate to be 146.2 g
Table 2: Calculation of percent composition of calcium in CaC2O4.H2O
Calculations show with the atomic mass of 40.8 g of calcium, molecular mass of calcium oxalate monohydrate (CaC2O4.H2O), the expected percent composition of calcium in unknown salt is 27.43%.
Data Sheet
The collected data from the experiment were recorded in the notebook and tabulated as shown below. Figures in the datasheet allowed the calculation of the mass of the product (CaC2O4.H2O) formed in the precipitation reaction of calcium cations and oxalate anions.
Mass of CaCO3 used in the preparation of stock solution = 1.239 g
Table 3: Masses measured of materials used and products in the first and second trials