119_1_Intro_07F

119_1_Intro_07F - 1 I. INTRODUCTION Analysis Quantitative...

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Unformatted text preview: 1 I. INTRODUCTION Analysis Quantitative Reactions Concentration Units Stoichiometric Calculations Harris, Chap 0,1,2. 2 Analysis Representative (random, composite) bulk sample Homogeneous laboratory sample Conversion of lab sample Check/eliminate interferences Determine concentrations in sample (replication) Interpret results (including uncertainties) 3 Analysis Volumetric (Titrimetric) Analysis Stoichiometric amount of titrant added ( from buret) : accurately known volume of accurately known concentration of titrant to react completely (exactly) with each sample. Titrations, rapid; solution preparation/standardization may be slow. Gravimetric Analysis Unknown (generally small) excess of precipitating agent added for quantitative precipitation of reagent and precipitate is weighed. Reagent concentrations need not be known accurately. Slow and tedious, but no standardization required. Instrumental Analysis Measure some property (absorbance, conductivity) proportional to concentration and determine concentration from calibration curve: A, vs. C. 4 Analysis Volumetric (Titrimetric) Analysis : V A *N A = meq(A) = meq (B) = V B *N B Gravimetric Analysis : Wt{X} = (Wt{Ppt})*(Grav Factor) Spectrophotometric Analysis : Beers Law: A = abC C = A/ab Wt{Sample} } C{solution } V{solution 100 Wt{Sample} (Wt{X}) 100 X % Wt : Both = = 5 Volumetric Analysis and Titrations Standard Solutions Accurately known concentrations Stable Rapid, quantitative reaction of known stoichiometry Detectable equivalence (end) point a. Direct preparation from primary standard (highest measurable purity, stable, easily handled, readily available, reasonable cost, high equivalent weight) b. Standardization against other primary standard 6 Gravimetric Analysis No standard solutions required, but high purity reagents are (often) needed. Slight excess of precipitating agent Quantitative reactions of known stoichiometry (Slow reactions, OK) Careful filtration and washing Dry (heat) to constant weight. 7 Instrumental Analysis Standard solution (accurate concentration of species) required. Prepare other solutions by dilution of standard solution with other reagents and measure property (Absorbance) to make calibration curve. Treat unknown as above, measure absorbance, determine concentration from calibration curve. 8 Quantitative Reactions Formation of a weakly ionized substance HC 2 H 3 O 2 (aq)+NaOH(aq)NaC 2 H 3 O 2 (aq)+H 2 O(aq) or in the net ionic form HC 2 H 3 O 2 (aq) + HO- (aq) C 2 H 3 O 2- (aq) + H 2 O K EQ = K A /K W = 1.75*10-5 /1.0*10-14 = 1.8*10 +9 [ ] [ ] [ ] A EQ K HO O H HC O H C K =-- 2 3 2 2 3 2 [ ] [ ] [ ] [ ] [ ] [ ] W W EQ K O H HC O H O H C O H K O H HC O H C K = = +- +- 2 3 2 3 2 3 2 3 2 3 2 2 3 2 9 Quantitative Reactions, Weak electrolyte Assume simple case: 0.10 mol HAc and 0.10 mol NaOH in 1 L of solution....
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This note was uploaded on 04/13/2008 for the course CHEM 119H taught by Professor Munson during the Fall '07 term at University of Delaware.

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119_1_Intro_07F - 1 I. INTRODUCTION Analysis Quantitative...

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