Determine If a Precipitate Will Form If Two Solutions Are Mixed

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Determine if a precipitate will form if two solutions are mixed what is Qsp? Qsp is an expression that is identical to the solubility to the solubility product constant. Why mixed solution has precipitate? When two solution mixed, the new solution will have a new Qsp for production (the calculate value which is calculated by concentrations of each kind of ions.), if this Qsp is greater than the Ksp of production, precipitate will form. In this kind of question, the concentration and volume of each solution usually be given. So we ne calculate each ion’s new concentrations in the new solution. We will use this formula: New concentration = c×V initialV final Then, we can calculate out the Qsp and compare with the Ksp Qsp <Ksp: the system attains equilibrium by moving to the right favouring dissociation. More solid can dissolve. Qsp = Ksp the system is at equilibrium. No more solid can dissolve. No precipitate forms. Qsp>Ksp The system attains equilibrium by moving to the left, favouring precipitation. A precipitate forms until equilibrium is reached. EXAMPLE: Problem: 1, 100ml of 1×10-3 mol/L Pb(NO3)2 is added to 40 ml of 0.040 mol/L NaCl. Does a precipitate form? What is given? We know the concentration and volume of the Pb(NO3)2 and NaCl for Pb(NO3)2, c= 1×10-3 mol/L and V= 100ml , for NaCl, c= 0.040 mol/L and V= 40 mol. Solution: we should write the balanced chemical equation first. Pb(NO3)2 + 2NaCl = PbCl2 + 2NaNO3 100ml=0.1L and 40ml= 0.04L newPb2+= 1×10-3 mol/L×0.1l 0.1l+0.04l=7.143×10-4mol/L new[cl-]=0.04mol/L×0.04L0.1L+0.04L =0.0114mol/L Qsp=Pb2+[cl-]2= 7.143×10-4mol/L×(0.0114mol/L)2=9.283×10-8 The Ksp of PbCl2 is 2.4×10-4 So we can see, the Qsp<Ksp so the precipitate doesn’t form. Example2: 25ml of 0.10mol/L

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