Lab 9 Determination of the Dissociation Constant of a Weak Acid (2).docx

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1Lab 9Determination of the Dissociation Constant of a Weak AcidUSF-SM, General Chemistry II LaboratoryTo become familiar with the operation of a pH meter and use pH to determine the magnitude ofthe equilibrium constant of a weak acid.ApparatuspH meter with electrodes250 mL beakers (2)100 mL beakers (2)Burette25 mL pipet and pipet bulbBurette clamp and ring standRing stand and ringRO water bottleChemicalsStandard buffer solution (pH = 4.00 and pH = 7.00 Buffer)Unknown solution of a weak acid0.1 M NaOHIntroductionAcid - Base EquilibriaAccording to the Brnsted- Lowry acid- base theory, the strength of an acid is related to its abilityto donate protons. All acid- base reactions are then competitions between bases of variousstrengths for these protons. For example, the strong acid HCl reacts with water according toEquation [1].HCl(aq) + H2O(l)H3O+(aq) + Cl-(aq) {1}This acid is a strong acid and is completely dissociated, in other words, 100% dissociated in diluteaqueous solution. Consequently, the [H3O+] concentration of 0.1MHCl is 0.1M.By contrast, acetic acid, CH3COOH (abbreviated HOAc), is a weak acid and is only slightlydissociated to form hydronium and acetate (abbreviated OAc-) ions, as shown in equation {2}.H2O(l) + HOAc(aq)H3O+(aq) + OAc-(aq) {2}Therefore, its acid dissociation constant, as shown by Equation {3}, is small.K=[?3?+][???]=1.8×105{3}a[????]Acetic acid only partially dissociates in aqueous solution, an appreciable quantity of undissociatedacetic acid remains in solution.
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Chapter 20 / Exercise 03
Experimental Organic Chemistry: A Miniscale & Microscale Approach
Gilbert/Martin
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For the general weak acid HA in aqueous solution, the dissociation reaction and dissociationconstant expression are,HA(aq) + H2O(l)H3O+(aq) + A-(aq) {4}K =[?3?+][?]{5}a][??Recall that pH is defined as pH = -log [H3O+] {6}Solving Equation {5} for [H3O+] and substituting this quantity into equation {6} yield[H3O+]= K][??{7}][?-log [H3O+] = -logKa-log][??[?]{8}pH = pKa-log][??

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Spring
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Dr.LauraAnderson
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Experimental Organic Chemistry: A Miniscale & Microscale Approach
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Chapter 20 / Exercise 03
Experimental Organic Chemistry: A Miniscale & Microscale Approach
Gilbert/Martin
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