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IB Chemistry Lab - Determining the Presence of Chemicals

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Introduction

Determining the Presence of Pb2+, Ag+ and Hg22+ Date Performed: 3/12/08 The molecular equations that involve Pb2+, Ag+ and Hg22+ with the addition of HCl can cause the formation of precipitate. These elements can be precipitated and separated with HCl and the addition of other ions. However, if HCl is in excess, the result may be the formation of soluble complex anions. Lead, mercury (I) and silver can be separated and identified by the addition of CrO4, the addition of NH3(aq) and acidifying the liquid using HNO3 respectively as well. Planning A Question: According to the composed equations from above, will Pb2+, Ag+ and Hg22+ be present in the unknown sample by using certain chemicals to separate these ions? ...read more.

Middle

- The following chemicals in drop bottles: o 6 M HCl o 1 M K2CrO4 o 6 M NH3 o 6 M HNO3 - a notebook to write down observations Procedure: Pour out a pea size amount of the unknown into a test tube while adding four drops of 6 M HCl. Stir well with a stirring rod and centrifuge so that the solution becomes thoroughly mix. Afterwards add a drop of 6 M HCl once more to the test tube and if the cloudiness forms within the test tube, then not all of the cations have precipitated. Because of this, centrifuge this solution once more after adding two drops of 6 M HCl. ...read more.

Conclusion

Afterwards, add three drops of 1 M K2CrO4 to the collected supernate and if a yellow precipitate forms, then Pb2+ is present. In order to identify whether or not Ag+ and Hg22+ is present in the unknown, add 10 drops of 6 M NH3 to the precipitate saved from the previous procedure and if a dark gray precipitate forms, then mercury is present in solution. Centrifuge this solution to separate the precipitate and supernate and decant the supernate, leaving the decantate in the test tube. Add 20 drops of 6 M HNO3 and stir well with a stirring rod. Take some litmus paper and check its acidity. If the solution is not very acidic, then continue to add 6 M HNO3 until it is acidic. If a white cloudiness precipitate forms, then Ag+ is present in the unknown. ?? ?? ?? ?? ...read more.

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