EXPERIMENT 8 : DETERMINATION OF Fe IN A FERUM SOLUTION BY GRAVIMETRIC ANALYSIS Abstract : This experiment is done to determine the percentage of ferum in a ferum solution. For the determination, we have used the precipitate technique to get the mass of ferum. In this part, gravimetric analysis is done by using filtration method to obtained the filtrate. From this part, the filtrate formed is poured into crucible and it is heated at 300 ̊C overnight to ensure that the filter paper have burnt completely to get the precipitate. The precipitate is weighed to calculate the percentage of ferum in a ferum solution. Objectives : 1. To Determine the Percentage of Ferum in a Ferum Solution Introduction : Gravimetric analysis is defined as based upon the measurement of mass that can be generalized into two types. Precipitation and volatilization are the types of this gravimetric analysis. The quantitative determination of a substance by the precipitation method of gravimetric analysis involves isolation of an ion in solution by a precipitation reaction, filtering, washing the precipitate free of contaminants, conversion of the precipitate to a product of known composition, and finally weighing the precipitate and determining its mass by difference. From the mass and known composition of the precipitate, the amount of the original ion can be determined. The successful determination can be obtained by substance desired must be completely precipitated. In most determinations the precipitate is of such low solubility that losses from dissolution are negligible. For example : Fe2+ Fe3+ + O2-
Fe3+ Fe2O3
The solubility of Fe 2O3 is reduced still further by the excess of Fe 3+ which is added, pushing the equilibrium to the right. The weighed form of the product should be of known composition. Since Fe 2O3 is a stable compound, the mass of Fe in the solution can be calculated.
Method : A. Precipitation
1.
A 25 mL Ferum solution is pipetted into a 400 mL beaker, 25 mL distilled water is added and it is covered with watch glass. The solution is heated until it is boiled. 1 mL concentrated nitric acid HNO3 is added by dropping it wisely. The solution is continued heating until it became clear. The solution is diluted to 200 mL with distilled water and it is boiled again. The beaker is transferred to the bench and 1 : 1 NH 4OH is slowly added into the solution and stirred until it is excessed. We might notice that is no more formation of precipitate and the steam odour is smelled.
B. Preparing a crucible 1.
The crucible is cleaned and it is heated for 10 minutes. It is cooled in desiccator and it is accurately weighed.
C. Filtering and washing the precipitate 1.
The precipitate is filtered by decantation using the precipitation method for gravimetric analysis without stirring the solution, so it ensured that most of the precipitate will be left behind in the beaker. The precipitate is washed with 30 mL warm 1% NH 4NO3 solution. The solution is poured into the filter funnel leaving behind most of the precipitate. The process is repeated for three times and finally all the precipitates are quantitatively transferred into the funnel. Finally, the precipitate is washed with warm distilled water and it is tested for chloride with acidic silver nitrate solution.
D. Drying and weighing the precipitate 1.
The filter paper that contained the filtrate is transferred into the crucible in above. The crucible is heated by using a Bunsen burner with a small flame at the beginning until all the paper burnt. The flame is increased and it is continued heating for another 20 minutes. The heating process is repeated until a constant mass is obtained. The position of crucible is recorded on the piece of paper at the furnace. The precipitate will be heated at 300 ̊C overnight. When the crucible is cooled, the mass of precipitate is weighed. The percentage of ferum is calculated.
Results : 1) Weight of crucible + lid + solid = 69.878 g 2) Weight of crucible + lid
= 67.898 g
3) Weight of solid(Fe2O3)
= 1.98 g
4) Percentage yield of ferum
= 91.85 %
Calculation : 1) Moles of Fe in sample = 25.0 ml *(1 L/1000 ML) * (60.4 Fe/1 L)*(1 mol Fe/55.85 g Fe) = 0.0270 mol Fe 2) Moles of Fe found experimentally, = mass solid Fe 2O3 * MW Fe2O3 = 1.98 g *(1 mol Fe 2O3 / 159.7 g Fe2O3) = 0.0124 mol Fe 2O3 From the equation, 2Fe 2+(aq) + 3/2 O2(g)
Fe2O3(s)
2 mol Fe ≈ 1 mol Fe 2O3 Mol Fe = (0.0124 * 2)/1 = 0.0248 mol Fe 3) Percentage yield of ferum, Fe = (actual experimentally mol Fe/mol Fe in sample ) * 100 = (0.0248 / 0.0270) * 100 = 91.85 %
Discussion : Before we entered the laboratory, we have made some preparation in the form of information from the expected data. The theoretical mole of Ferum in sample is 0.027 mol Fe. From that mole, the exact percentage of Ferum can be determined. As we compare it to our data, the experimental mole of Ferum is 0.0248 mol. From this mole, we have calculated the percentage yield by using the actual yield (mole of Fe). The percentage yield is 91.84%. as we made a comparison between mole for the both of theoretical and experimental, we believed in our data that we have obtained experimentally is compatible with the theoretical data.
Conclusion : 1. The percentage of Ferum in Ferum solution obtained is 91.85%. References : 1) HARGIS L.G, Analytical Chemistry , principle and techniques, 1998 2) www.ferum.wordpress.com
