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Using precipitation to determine unknown species in a solution

Using precipitation to determine unknown species in a solution. Today we are going to determine which unknown cations we have in solution using precipitation. To begin can you think of why precipitation would be a useful method for determining if we have a cation in our solution?

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Using precipitation to determine unknown species in a solution

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  1. Using precipitation to determine unknown species in a solution Today we are going to determine which unknown cations we have in solution using precipitation. To begin can you think of why precipitation would be a useful method for determining if we have a cation in our solution? Can you think of why precipitation would be particularly useful if we have several different unknown cations in our solution?

  2. As a warm up, show what chemical species will be in each of our solutions For example: NaBr(s) + H2O(l) Na+(aq) + Br-(aq) + H2O(l) ,so we have Na+ and Br- in solution NaCH3COO (aq) KBr (aq) NH4F (aq) Na2CO3 (aq) K3PO4 (aq)

  3. Let’s say we were to take an aliquot (a small sample) of NaCH3COO(aq) solution and an aliquot of Al(Br)3 solution, we combine the two solutions. Using the solubility table on the last page please write the balanced chemical equation for the reaction making sure to note if the products are aqueous or solid. Please draw all of the chemicals in your reaction in the beaker below. Chemical Equation: NaCH3COO (aq) Al(Br)3(aq)

  4. Proceedure: • Get an unknown solution and 5 test tubes from the front of the room. • Place half of your unknown solution in a test tube. • Add 10 drops NaCH3COO (Sodium Acetate) to the unknown solution you placed in the test tube. • Did a precipitate form? • Yes • Centrifuge and pour the solution (decant) into a new test tube. • Fill in your chart with the identity of the precipitate (ppt) and the cations still possibly in your solution. • No • Fill in your chart with “None” for the ppt and the possible cations still in your solution. • Add 10 drops of KBr solution to your decant. • Repeat step 4 • Add 10 drops of NH4F solution to your decant. • Repeat step 4. • Add 10 drops of Na2CO3 solution to your decant. • Repeat step 4. • Add 10 drops of K3PO4 solution to your decant. • Repeat step 4.

  5. Unknown # Possible Unknown Al3+, Ca2+, Cu3+, Pb2+, Li+ CH3COO- ppt decant Br- decant ppt F- decant ppt CO32- decant ppt PO43-- ppt decant Cations in solution: ___________________________________________________________

  6. Lab Follow Up Questions Having completed the lab which unknowns did you have in your solution? Why was precipitation helpful in making this determination?

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