4. In step 4 of the procedure which is (Wash solid A (Ag+ , Pb2+) by filling the

Question

4. In step 4 of the procedure which is (Wash solid A (Ag+ , Pb2+) by filling the test tube about ¼ full with hot water.  Mix/stir thoroughly and then allow the precipitate to settle. Thorough mixing is very important to making sure the Ag+ and Pb2+ are separated. Use a clean micropipet to carefully remove the liquid without disturbing the precipitate and place in another clean test tube. Repeat this washing process a second time and transfer the washing to the same test tube asthe first washing (combine both wash solutionstogether in a single test tube).  Label the test tube and set aside – the tube with the two combined washings is solution B (Pb2+) in the flowchart), solid A contains chloride precipitates of Ag+ and Pb2+.  However, the use of hot water allows the PbCl2 solid to be redissolved, but not the AgCl precipitate.

a. Based on general solubility rules, should PbCl2 dissolve in water?  Explain.

b. The solubility at 25C degree for AgCl  = 1.9x103 g/L and for PbCl2 = 4.5 g/L.  Assuming that the solubility for both substances increases as the temperature increases, explain why you think it is reasonable for the PbCl2 precipitate to selectively redissolve at higher temperatures.

Explanation / Answer

a) PbCl2 is water insoluble. However if solution heated to certain temperature, the chloride bonds break of and attach to the hydrogen of polar H20 molecules. This is possible because of weak hydrogen bonds, chlorine withdrawing hydrogens from the water.

b) If Ksp> Product of reactants, it dissolves , in the opposite case it precipiatates, for the case of AgCl Ksp is much lower and hence precipitation occurs.

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