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Research paper example essay prompt: Qualitative Analysis - 1060 words
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.. ed. First, HCl was added to the solution. There was no reaction, so H2SO4 was added to another sample of the unknown solution. This also resulted in no reaction, so NaOH was added to another sample of the solution. A rusty-brown precipitate appeared, which meant either PO43- or Cu2+ was present.
To determine which one of these ions was present, NH4+ was added to the solution, and a rust-colored precipitate formed. This confirmed the presence of Fe3+. Next, the unknown had to be tested for anions. The anion flow chart was followed. 2 drops of unknown were reacted with Ba2+, and there was no reaction.
2 more drops of unknown were reacted with Ag+, and white and tan precipitates formed. H2SO4 was added to the test tube containing the precipitates, and a white precipitate was left. This confirmed that PO43- was present but dissolved when the H2SO4 was added (as was found in part 1 of the experiment), leaving the Cl-. Therefore, the anions present were Cl- and PO43-. Data and Calculations: The data charts and the flow charts are on the following pages.
Unknown #2 contains Fe3+, Cl-, PO43-. Net ionic equations for precipitates and reactions on flow charts: Ag+ + Cl- AgCl Ba2+ + SO42- Ba SO4 Fe3+ + 3OH- Fe (OH)3 Cu2+ + 2OH- Cu (OH)2 Cr3+ + 3OH- Cr (OH)3 3Ag+ + PO43- Ag3(PO4) Ag3(PO4) + H2SO4 Ag2(SO4) + H3PO4 Thought process for flow charts: 2 separate flow charts had to be made, one for the cations and one for the anions. Starting with the cations, the flow chart must begin by listing all cations possibly present because the unknown can contain any number of them. HCl was the first reagent added on the flow chart because it only produced a precipitate with one of the cations, Ag+. This was determined using the data chart from part 1 of the experiment, where the precipitates formed with each reagent were clearly delineated.
By beginning the flow chart with reagents that produce fewer precipitates and ending it with the ones that produce more, the chart was easier to follow during the testing for ions. Therefore, the next reagent used on the chart was H2SO4. Since the Ag+ precipitated out as AgCl, the ions left to react with H2SO4 were Ba2+, Fe3+, Cu2+, and Cr3+ (these were the ions that were left unreacted by the HCl). By referring to the data chart from part 1 of the lab, it was found that H2SO4 only formed a precipitate with Ba2+(BaSO4). This meant that the Fe3+, Cu2+, and Cr3+ ions were left unreacted. NaOH was added to these ions and it formed a rust-colored precipitate with Fe3+, while it formed a blue precipitate with Cu2+.
To confirm whether the precipitate was from Fe3+ or Cu2+, NH4+ was added to the unknown. If a rusty-brown precipitate appeared, the ion present was Fe3+(Fe(OH)3). If no precipitate formed but the solution turned dark blue, Cu2+ was present. The only ion left unreacted after the NaOH was added was Cr3+. If a bluish-white precipitate formed when NH4+ was added to the Cr3+, it confirmed the presence of Cr3+(Cr(NH4)3).
The anion flow chart began with all three of the anions listed (SO42-, Cl-, PO43-). Ba2+ was the first reagent added because it formed a precipitate with only one of the ions, SO42-(BaSO4). The ions left unreacted were Cl- and PO43-. Ag+ was added to these. At this point, a white precipitate could form with Cl-(AgCl), or a yellow precipitate could form with PO43-(Ag3(PO4)).
If the precipitate was purely white, the ion was Cl-. This was confirmed by adding H2SO4 to the precipitate, which would result in no reaction. This is because H2SO4 does not dissolve AgCl (from part 1 of the experiment). Then, the addition of NH4+ would dissolve the precipitate and prove that only Cl- was present. However, if the unknown contained both Cl- and PO43-, both precipitates would form, but the two colors of the precipitates would be indistinguishable from each other. A yellow precipitate would be seen, but it would be impossible to tell if there was also a white precipitate present. Therefore, H2SO4 would again be added to the precipitates.
If a white precipitate appeared, it would mean that PO43- had been present, but it was dissolved by the H2SO4. It would leave only the AgCl precipitate visible (H2SO4 dissolves Ag3(PO4), but not AgCl), but because the precipitate originally had a yellow color to it, it would be known that both the Cl- and the PO43- ions were present in the unknown solution. If the solution had no precipitate left (turned clear) when the H2SO4 was added to the yellowish precipitate, it would indicate that only PO43- was present in the solution. This is because H2SO4 dissolves Ag3(PO4), and there were no other precipitates left in the solution to be seen. Results and Discussion: In conclusion, unknown #2 contained Fe3+, Cl-, PO43-. This was determined using qualitative analysis, and the purpose of the experiment was therefore fulfilled. One possible source of error for this lab could occur in part 1, where the reactions of different reagents with different ions are recorded in data charts.
If there is incorrect information about whether or not a precipitate formed, it will most likely result in an incorrect flow chart and an incorrect identification of the ions in the unknown. That is why it is important to use the centrifuge if there is uncertainty about a precipitate, or the reaction should be performed again. Another source of error would be to add too many drops of reagent to the ion or sample of unknown. This is because a precipitate may form with the reagent, but dissolve in an excess of the reagent. Therefore, the precipitate could form but then be dissolved without ever being seen.
This would also result in an incorrect flow chart and an incorrect identification of the unknown ions. Another source of error could occur in making or following the flow chart. Incorrect reasoning when designing the flow chart will make it difficult to correctly identify the ions in the unknown, and not following the flow chart correctly would obviously cause error in the final results.
Research paper topics, free essay prompts, sample research papers on Qualitative Analysis