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Lecture06 - 6017 exec{al OB s-1 98 17 dp 6 p-1 CA l...

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Ba(IO3)2(s) . currentpoint currentpoint Ba2+ + 2IO3192837465 S 2S S -----> Ksp = [Ba2+][IO3-]2 = 1.57 x 10-9 M3 S(2S)2 = 1.57 x 10-9 M3 4S3 = 1.57 x 10-9 M3 S = 7.32 x 10-4 M = Solubility of Ba(IO ) 3 2 mmol Ba(IO3)2 = (7.32 x 10-4 M)(500 mL) = 0.366 mmol mg Wt Ba(IO3)2 = 0.366 mmol x 487 mmol = 178 mg = 0.178 g Common Ion Effect: The presence in solution of an excess of one ion from a slightly soluble ionic substance depresses its solubility (more complete precipitation). Example: Solubility of Ba(IO3)2 in the presence of 0.0200 M Ba(NO3)2? Ba(IO3)2(s) . currentpoint currentpoint Ba2+ 192837465 + 2IO3- S -----> S + 0.0200 2S [Ba2+][IO3-]2 = 1.57 x 10-9 M3 (S + 0.0200)(2S)2 = 1.57 x 10-9 M3 *Assume S << 0.0200 M, then: (0.0200)(2S)2 = 1.57 x 10-9 M3 S = 1.40 x 10-4 M = Solubility of Ba(IO ) 3 2 Theory of Precipitation Precipitation occurs when the ion product exceeds the solubility product. For MxAy: CMxCAy > Ksp Two Steps in Precipitation 1. Nucleation: small groups of ions form stable solid masses (nuclei). 2. Particle Growth: ions from solution add on to nuclei. Supersaturation: When ion product exceeds Ksp, instantaneous con (Q) exceeds equilibrium concentration (S). Large Supersaturation favors nucleation. Formation of new (small) particles. Small Supersaturation favors growth. Existing particles get larger. Supersaturation minimized by: 1. Elevated temperature (incr S) 2. Dilute solutions (decr Q) 3. Slow addition, good stirring (decr Q) Colloids: very small particles which will not settle and cannot be filtered. Coagulation: aggregation of colloidal particles into larger masses which will settle and can be filtered. Add salt to screen surface charge. Stir and/or heat. Skoog, West and Holler Figure 8-2 Page 184 Impurities in Precipitates (Coprecipitation) Adsorption: Excess precipitant ions, and counterions, adsorbed on crystal surface. Small particles = Large surface area = More impurities adsorbed Mixed-Crystal Formation: A normally soluble compound precipitates in the presence of an insoluble compound. Occlusion and Mechanical Entrapment: Ions from counter-ion layer or pockets of liquid trapped inside crystal. Purification of Precipitates 1. Digestion: Allow precipitate to remain in contact with solution from which it was formed at elevated temperature. Small particles dissolve faster Ions reprecipitate on larger particles Increases purity 2. Wash: Soluble impurities removed by washing. Not always desirable. Peptization: Reversal of coagulation caused by washing away coagulating ions. Overwashing: All compounds have some solubility. Precipitation from Homogeneous Soln One of the precipitating ions is generated by slow chemical reaction. Urea produces hydroxide for M3+ det. See text Hydroxylamine produces Cu+. See Expt 4 Sulfamic acid produces sulfate for M2+: HSO3NH2 + 2H2O -----> SO42- + NH4+ + H+ SO42- + M2+ -----> MSO4(s) M = Ba, Ca, Sr, Pb Fundamental Relationship of Titrimetric Analysis At the Equivalence Point 1) Direct Titration: moles of titrant = moles of substance being titrated x stoichiometric ratio 2) Simultaneous Titration: moles of titrant = moles 1 x S.R. 1 + moles x S.R. 2 2 3) Back-Titration: Amount E = Amount A x S.R. + Amount B x S.R. A B 4) Sequence of Reactions: Start at end (analytical rxn) Work backwards ...
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