Deterination of Molar Mass and Ionization Constant of a Weak Acid (1).pdf

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28-1PurposeDetermine the molar mass and acid dissociationconstant of acetic acid (a weak monoprotic acid)by titration and pH measurement of solutionscontaining a known concentration of acetic acid.Determine the molar mass and acid dissociationconstant of an unknown weak acid.Learn how a pH meter works and how to measurethe pH of a solution.Pre-Lab PreparationAcids and bases are important classes of chemicalcompounds. They control the pH of living systemsand of many reactions carried out in the chemicallaboratory. If the pH of the blood of most animalsshifts by as little as 0.3 units above or below the nor-mal range of 7.3 to 7.5, severe illness results. There-fore, if we understand the principles of how acidsand bases function, we will be better informed aboutthe functioning of biological systems, as well as ofpurely chemical systems.In this experiment, we will study the propertiesof weak acids that can ionize (dissociate) by reactionwith water to yield one hydronium ion (a protonatedwater molecule, H3O). Such acids are calledweakmonoprotic acids. Acetic acid, the active ingredientin vinegar, is a common weak acid belonging to thisclass. We will determine its molar mass by titrationand its ionization constant by making measurementsof pure acetic acid solutions and a half-neutralizedsolution of acetic acid. We can also apply exactly thesame principles to determine the molar mass andionization constant of an unknown weak monoproticacid.Acetic acid has the molecular formulaHCCHHOOHThe molecular formula for acetic acid is generallyshortened to read CH3COOH or HC2H3O2. Thesechemical formulas are often further abbreviated toHA, where HA represents acetic acid (or any genericweak monoprotic acid). Then the symbol Astandsfor acetate ion, CH3CO2, the anion of acetic acid (orthe anion of any weak monoprotic acid, HA).The proton that dissociates in acetic acid is theone attached to the oxygen atom. The OH bond inthe acetic acid molecule is appreciably weaker thanthe CH bonds, and the polarity of the OH bondmakes it easier for the proton to transfer to a watermolecule. In contrast to the behavior of hydrogenatoms bonded to oxygen, there is no significant ion-ization of the hydrogen atoms bonded to carbon inaqueous solution.Acetic acid ionizes according to the reaction(1)The equilibrium constant expression is given by(2)The literature value of the equilibrium constant forthis reaction is 1.76105at 25°C.1You are tomeasure the value of this constant in this experimentand compare your value with the literature value.A straightforward way to obtain the value ofKaisto measure each of the quantities on the right-handside of Equation (2) and carry out the indicated arith-metic. For solutions containing only the weak acid HA,the value ofKacan be determined by examining theionization reaction shown in Reaction (1) and notingthat the concentration of Awill be equal to that ofthe H3O(ignoring the insignificant amount of H3Othat comes from the ionization of water). The HAKaCH3OD CADCHADHAH2OTH3OAE X P E R I M E N T28Determination of the Molar Massand Ionization Constant of a Weak Acid1This is the value ofKaobtained by extrapolating to zero ionicstrength. In any real solution of acetic acid, the ionic strength isgreater than zero, and the dissociation constant is somewhatlarger because of electrostatic interactions between the ions in

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