Lec_05 - Today 1 Report for Experiment 3 is due(40 pts 2 Post-lab for Experiment 3 is due(10 pts 3 Unknown Summary Sheet for Experiment 3 is due(20

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Unformatted text preview: Today 1. Report for Experiment 3 is due (40 pts) 2. Post-lab for Experiment 3 is due (10 pts) 3. Unknown Summary Sheet for Experiment 3 is due (20 pts) 4. Pre-lab for Experiment 4 is due (5 pts) 5. Experiment 4. Acid-Base Titration have a write-up in your notebook Next week 1. Report for Experiment 4 is due (40 pts) 2. Post-lab for Experiment 4 is due (10 pts) 3. Unknown Summary Sheet for Experiment 4 is due (20 pts) 4. Pre-lab for Experiment 5 is due (5 pts) 5. Experiment 5. Synthesis and Analysis of a Complex Iron Compound. Part 1 have a write-up in your notebook Spring 2007 AcidExperiment 4. Acid-Base Titration PURPOSE AND LEARNING OBJECTIVES To learn the technique of standardizing a solution To identify the unknown acid using qualitative analysis To determine the concentration of acid in an unknown sample using acid-base titration (quantitative analysis) To analyze the acid content of a fruit juice Spring 2007 Volumetric Titrations A volumetric titration is a quantitative method of analysis in which the titrant is added to an analyte and the volume of the titrant required to complete the reaction is measured Titrant a reactant of known concentration Analyte a reactant of unknown concentration Measure: the volume of a titrant (ml) Determine: concentration of an analyte (M) Spring 2007 Volumetric Titrations Most common reactions used in titration: acid-base oxidation-reduction complex formation precipitation reaction Principal Requirements for a titration reaction: proceeds rapidly proceeds to completion must know the reaction stoichiometry Spring 2007 The Equivalence Point The equivalence point occurs when the quantity of added titrant is the exact amount necessary for stoichiometric reaction with analyte (theoretical) Example: A = analyte B = titrant A + 2B products At the equivalence point: # of moles of A = 1 mole of A 2 moles of B 1 # of moles of B 2 Spring 2007 AcidStrong Acid-Strong Base Titration Example: HCl + NaOH NaCl + H2O At the equivalence point: # of moles of HCl = # of moles of NaOH OH = titrant H+ = analyte [NaOH] = 0.328 M VNaOH = 24.58 ml 0.328 mole/L 24.58 ml Moles of NaOH = = 0.00806 mole 1000 ml/L At the equivalence point: # of moles of HCl = # of moles of NaOH = 0.00806 moles VHCl = 5.00 ml [HCl] = ? M HCl = 0.00806 mole 1000 ml/L = 1.61 mole/L 5.00 ml Spring 2007 How Do We Know When the Equivalence Point is Reached? The end point is the stopping point of titration marked by a sudden change in a physical property of solution (experimental): change in the voltage or current change in pH change in the absorption of light An indicator is a compound that undergoes an abrupt change in color near the equivalence point Spring 2007 AcidEnd Point in Acid-Base Titrations A number of indicators is available We will be using phenolphthalein: Colorless in acidic solutions Violet in basic solutions Change point is at pH = 8.2 acidic pH < 8.2 basic pH > 8.2 end point pH 8.2 Spring 2007 Why Standardize? To titrate an acid of unknown concentration, we need to know the exact concentration of our titrant NaOH solution. Experimental complications: NaOH is hygroscopic NaOH reacts with CO2 dissolved in water: CO2 (g) + H2O (l) H2CO3 (aq) 2 NaOH (aq) + H2CO3 (aq) Na2CO3 (aq) + 2 H2O (l) The NaOH solution of exact concentration cannot be prepared by simply dissolving a weighed amount of pure NaOH in a known volume of solution Spring 2007 How to Standardize? Prepare NaOH solution of an approximate concentration Standardize NaOH solution with a primary standard standard solution Primary standard Potassium Hydrogen Phthalate, KHP (acts as an acid) H H OK OH H H O O H H O O_ O_ + NaOH H H O + Na+ + K+ + H2O At the equivalence point: # of moles of KHP = # of moles of NaOH Primary standard prepared by dissolving a weighed amount of pure reagent (99.9% or better) in a known volume of solution Spring 2007 Miscellaneous Tips Tips Weigh NaOH pellets before dissolving them in water Indicate the mass of KHP on each flask Read the burette to 0.01 ml Do not contaminate your original unknown acid solution (always pour out portions of the unknown acid solution into a dry clean beaker) Do your calculations before discarding leftover NaOH solution Keep your unknown acid solution in your laboratory drawer until you receive your graded Titration Unknown Summary Sheet back Spring 2007 ...
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This note was uploaded on 11/10/2008 for the course CH 204 taught by Professor Leytner during the Spring '08 term at University of Texas at Austin.

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