IB Chemistry Lab - Determining the Presence of Chemicals

Hypothesis: I believe that Pb2+, Ag+ and Hg22+ will be present in the unknown if the composed equations are correct. According to the equations, a solid forms after the addition of certain chemicals such as ammonium.

Planning B

Variables:

• An unknown sample
• A centrifuge machine
• Three small test tubes
• About 500 mL of distilled water
• A medicine dropper
• A ring stand
• An iron ring
• A wire gauze
• A stirring rod
• A beaker
• Several pieces of litmus paper
• A Bunsen burner

• The following chemicals in drop bottles:

• 6 M HCl
• 1 M K2CrO4
• 6 M NH3
• 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. Next decant the solution and dispose of the supernate. Next add a few drops of distilled water into the test tube and stir with a stirring rod, then centrifuge afterwards. This is to wash the precipitate. After centrifuging, dispose of the liquid again, leaving the precipitate in the test tube. Next, to identify the presence of Pb2+ perform a hot water bath using a ring stand, an iron ring, wire gauze, a Bunsen burner and a beaker. Fill the beaker halfway with water and add 15 drops of distilled water to the test tube, placing the test tube in the hot water bath afterwards. While in the hot water bath, stir well with a stirring rod for about a minute and centrifuge. Decant the supernate into a new test tube afterwards. Do this procedure twice in order to collect supernate into the test tube. 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.