By: Howard Andres

In this laboratory experiment, our lab group’s purpose was to determine the percentage of iron in an unknown containing iron(II) ammonium sulfate by titrating it with potassium permanganate, KMnO₄.  By finding out the volume of KMnO₄ used in the titration, we were able to obtain, through calculation, the moles of KMnO₄ required, the moles of Fe²⁺ in the sample, the mass of Fe in the sample, and the percentage of Fe in the sample. The experiment was carried out in three trials.

The titrations were carried out by filling a 50 mL buret with .0195 M KMnO₄ and allowing it to flow into a beaker containing about one gram of the dissolved sample in diluted H₂SO₄.  Once a transparent light-yellow color developed after putting some KMnO₄ into the solution, we were to add 3 mL of 85% H₃PO₄.  Doing so allowed the solution to make its yellow color disappear and have the solution become clear again.  As the titration continued a pink color developed which quickly appeared then disappeared.  The titration ended once the light pink colored stayed in the solution for at least fifteen seconds.

In the first trial, 16.4 mL of the KMnO₄ was used to titrate the solution with 1.05 g of the sample.  With this  amount, 3.20 x 10^-4 mol of KMnO₄ was found to be required for titration.  0.00160 mol of Fe²⁺ was in the sample, and the mass of Fe in the sample was .0894 g, with it having a percentage of 8.51%.  In the second trial, 11.4 mL were used to titrate 1.00 g of the sample, meaning that 2.22 x 10^-4 mol of KMnO₄ were required, 0.00111 mol of Fe²⁺ were found in the sample, and .0621 g of Fe were in the sample.  It had a percentage of 6.21%.  The final trial required 12.0 mL of KMnO₄ to titrate 1.03 g of the sample.  Thus, 2.34 x 10^-4 mol of KmnO₄ were required, and there were 0.00117 mol of Fe²⁺, 0.0653 g of Fe, and 6.34% of Fe in the sample.

Overall, the experiment proceeded quite well.  Some error can be noted though in the first trial were we might of over titrated a little bit since the results in the calculations were more off when compared with the other two trials, and the end point had a little darker pink color than the other two trials.  Thus, with the greater amount of KMnO₄ used, the moles, mass, and percentage of Fe in the sample were affected.  Additionally, a minor spill occurred when pouring the H₂SO₄ in the beaker for the third trial.  Our group added a few milliliters of H₂SO₄ into the beaker to compensate for what was lost.  Even though the results came out to be similar to trial two, there was this error and thus the results of the two, despite being close, were not as close as they could have been.  Additionally, the different mass amounts of the solid used in each trial could have affected the results since with more of the solid, the more amount of KMnO₄ was used for the titration. Despite these points though, the experiment went well.

[ Back ] [ Up ]