Why is it not necessary to test the supernatant from both trial samples for the excess reactant is procedural step number 9? Figure 6.1 Opened filter paper to place into filter funnel.7. Stir up your reaction vigorously before filtering. Decanting the liquid into the filter paper firstwill actually result in most of the solid flowing through the pores of the paper.8. Filter the first sample through the first filter paper. Transfer all of the precipitate by using arubber policeman (rubber or TeflonTM spatula) and no more than three 5 mL aliquots of thewarmed wash water. Be sure to collect as much of the precipitate as possible. When thefiltration is complete, carefully remove the filter paper to a clean watch glass for drying in asafe place until the next laboratory period.9. Use the plastic transfer pipet to half-fill each of the two centrifuge tubes with the liquidfiltrate. Only centrifuge the samples if necessary. Label the tubes I and II. Add 2 drops of BaCl2test reagent to tube I, and 2 drops of Na3PO4 test reagent to tube II. You should see an obviousprecipitation reaction in only one of the tubes.10. Discard the filtrate (the liquid in the receiving flask) from the first sample into the liquid wastecontainer and rinse the flask 3 times with deionized water. Use the second filter paper tofilter the second sample in identical fashion to the first.11. Remove the filter paper from the funnel, unfold, and place on a watch glass. The sampleshould be placed in the oven for about 15 minutes to dry.12. Record the mass of the dried filter paper and precipitate.WasteAll liquid waste can go in the aqueous waste container. Solid waste can be placed in the solidwaste container.Clean upRinse all glassware thoroughly with deionized water. Dry all glassware and return to theproper place. Materials:Bunsen BurnerRing standWire mesh100 mL graduated cylinder (2)250 mL beakers (2)150 mL beakerTest tubes (2)Test tube rackGlass stir rods (2)Filtering funnelPlastic transfer pipet250 mL Erlenmeyer flaskNote: You will run two samples simultaneously. Perform the procedures for both samples.Obtain approximately the same volume of the same unknown samples for both trials.1. Label the graduated cylinders with labelling tape. One will be used for the BaCl2 solution andthe other for the Na3PO4 solution.2. Obtain approximately 20 mL each of an unknown BaCl2 and Na3PO4 solutions. Record thevolumes (to the correct number of significant figures) in your notebook.3. In the 250 mL beaker, combine the 20 mL samples of BaCl2 and Na3PO4 solutions and 100 mLof deionized water. Stir each trial solution for 3 minutes. When finished stirring, remove thestir rod from the beaker, cover with a watch glass, and allow the precipitate to settle.4. While the precipitate is settling, set up the ring stand with wire mesh, light the Bunsen burner,and adjust the flame to a blue flame (with no traces of yellow). Place one sample on the wiremesh and digest the precipitate for 15 minutes. Do not let the mixture come to a boil! Thegoal is to simply gently heat the reaction mixture. Remove the first sample from the ring standafter 15 minutes and digest the second sample for 15 minutes.5. After both reactions have finished digesting, heat about 100 mL of deionized water to 70-80°C in the 150 mL beaker. This water will be used for washing the precipitate out of the beakersand for washing precipitate off of the stir rods.6. Set up the filtration apparatus. Obtain 2 sheets of fine porosity filter paper. Using pencil, writeyour initials and sample number on each paper. Fold each piece of filter paper into quarters.Then, open the filter as shown in Figure 6.1. Place the first filter paper in the filter funnel, andfix it there by wetting it with some deionized water. Allow the deionized water to flow intothe 250 mL Erlenmeyer flask and discard the water.

In step 9, BaCl2 is added to test tube 1, and Na3PO4 is added to test tube 2. Precipitation occurs in only 1 of the test tubes.This happens because both the test tubes will have the excess reactant in the supernatant liquid and that excess reactant will react with the other one to cause precipitation. Let's see this as an example if BaCl2 is the excess reactant. In test tube 1, if BaCl2 is added, no reaction occurs. But in test tube 2, BaCl2 is present already and Na3PO4 is added then precipitation occurs. So, we are actually testing the supernatant from both trial samples for the excess reactant is procedural step number 9.