Gravametric Quant Lab Report

The Gravimetric determination is the measurement of mass in two different forms precipitation and volatilization. In our experiment, we will be using the precipitation form which isolates an ion in a solution by a precipitation reaction, filtering, purifying by wash method, conversion to the product of known composition, and final weigh of the product comparing the mass difference of theoretical and actual. This method identified weight percent of iron in an unknown sample. Three samples are taken to limit the percent error. In the results of the three samples 1 had a percent of 10. 764 Fe (III), sample 2 had a percent of 11. 725 Fe (III), and sample 3 with a percent of 12. 216 Fe (III). The average sample percent was 11. 568 compared to the given amount percent of 12. 90. In theory with a loss of 1. 332 this experiment was overall successful. Introduction: In this lab, the purpose was to use the gravimetric determination procedure to identify the weight percent of iron in an unknown sample. Three samples were collected and analyzed.

Iron can be analyzed by precipitating the hydrated iron oxide from a basic solution. After the basic solution is hydrated the process is then followed by complete dehydration to give solid iron oxide. Methods and Materials: Needed in the experiment was; Crucibles, Metal rings, Wire triangles, Burners, Funnels, Filter Paper, Beakers, Glass rod, Diluted ammonium hydroxide solution, Nitric acid solution, Silver nitrate solution, NH4NO3 solution, Distilled water. Below are some methods used in the experiment. fig. 1 fig.

2 Experimental Procedure: This experiment was a multiple session lab.

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Obtain three crucibles and desiccator. Bring the three porcelain crucibles and caps to constant mass by heating to redness for 15 minutes over a burner, use fig. 1 for method reference. Place the heated crucibles in the desiccator to cool for approximately 30 minutes and weigh. This was left overnight and completed the second trial in the next session with successive weighing agreed within 0. 30mg. (Keep constant numbering with crucibles throughout the experiment) We measured out 1. 5g of three samples of the unknown that was given to us. Each sample was dissolved in 10 mL of 3M HCl (with heating necessary). mL of 6 M HNO3 was obtained to the filtrate, and boil for a few minutes to ensure that all iron is oxidized to Fe (III). The samples were diluted to 200mL with distilled water and add 3 M ammonia with constant stirring until the solution was basic (as determined with pH indicator paper). After the solution becomes basic, digest the precipitate by boiling for 5 minutes and allow the precipitate to settle. We then decanted the supernatant liquid through coarse, ashless filter paper (Whatman 41 or Schleicher and Schuell Black Ribbon, as in fig. 2 -18 and 2 -19 in the textbook. ). Keep liquid lower than 1 cm from the top of the funnel.

Our precipitate was first washed repeatedly with hot ammonium hydroxide solution, by miscommunication. Then washed with the corrected heated ammonium nitrate and left it to drain overnight until the next session. We continued to wash supernatant until little or no Cl- is detected in filtered supernatant. Detect the Cl- by acidifying a few milliliters of filtrate with 1 mL of dilute HNO3 and adding a few drops of 0. 1 M AgNO3. If a precipitate is observed, Cl- is present. After identifying that there was not any Cl- present we allowed the filter to drain overnight covered with ventilation. Carefully, the paper was lifted out of the funnel, folded (fig. 2), and transferred all dried substance to the crucible and any substance that is not completely dry place into a beaker and into the heater for half an hour. Those placed in beaker were then placed into the crucibles that were brought to constant mass. With the paper and substance in the crucible, it was placed over a small flame with the lid off to start to char the filter paper. The flame temperature was then increased keeping the lid handy to smother the crucible of the paper flames.

After the paper seems visibly charred ignite the product for full 15 minutes with full heat of the burner directed at the base of the crucible where oxidized iron is located. When the crucibles have briefly cooled in the air, we then placed them in the desiccator for 30 minutes. After the 30 minutes of cooling in the desiccator weigh the crucible and the lid, reignite, and bring to constant mass with the repeated heating within a mass of 0. 3 mg. We are now complete with the experiment. Calculate the weight percent of iron in each sample, the average, the standard deviation, and the relative standard deviation for your data.

Results: Crucible 1: 0. 231 g Fe2O3 ? 1 mol Fe2O3159. 487g ? 2 mol FeOOH1 mol Fe2O3? 55. 845 g1 mol = 0. 162g Fe0. 162 g1. 505 g? 100=10. 764% Crucible 2: 0. 252 g Fe2O3 ? 1 mol Fe2O3159. 487g ? 2 mol FeOOH1 mol Fe2O3? 55. 845 g1 mol = 0. 176g Fe0. 176 g1. 501 g? 100=11. 725% Crucible 3: 0. 268 g Fe2O3 ? 1 mol Fe2O3159. 487g ? 2 mol FeOOH1 mol Fe2O3? 55. 845 g1 mol = 0. 183g Fe0. 183 g1. 502 g? 100=12. 216%. Refer to appendix for sample mass table and calculation equations

Discussion: Since the obtained and expected results are not 100 percent match we can conclude that during the experiment we encountered a loss of product, with an average percent of 11. 57 and an obtained of 12. 90 percent. In the experiment the precipitate was washed repeatedly with a given solution to filter out any Cl- at this time we notice that some of the precipitates had gone through the filter through the sides from the solution being held to high causing an overflow on the sides of the filter.

This was noticed by the orange tint in the beaker of the filtered solution. In the experiment, scales were also changed due to overuse. That could cause some flux in the measurement changes to a small degree. Another error or issue during the experiment lids on our crucible broke having to replace it caused a changed in our final weigh being that in the beginning, we weighed our crucibles with the lid. Remaining constant in the lab is a must this does cut back on experimental error such as using the same analytical balances and labeling all equipment and crucibles. The Gravimetric determination is the measurement of mass in two different forms of precipitation and volatilization. Some of the underlying principles and theories of gravimetric analysis are the law of mass action, reversible reactions, and the principle of solubility product and common ion effect.

Conclusion: The gravimetric determination procedure determined that we had an average of 11. 568% of Fe in our unknown solution, given the amount of 12. 90% of Fe. We experienced a loss of approximately 1. 332 %. This loss could be included in instrumental and human errors.

References

1. Lewis, D. 2013.
2. Quantitative Analysis Lab Journal. Gravimetric Determination of Iron as Fe2O3. Vol. 1: Pages 4 – 5. Franklin, J. 2013.
3. Quantitative Analysis Lab Journal. Gravimetric Determination of Iron as Fe2O3. Vol. 1: Pages 7 - 11. Bb Learn. 2013.

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