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Exp.9 - Chem Post Lab 2.docx - Experiment 9: Determination...

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Experiment 9: Determination of Acid Dissociation Constant03/26/2020Introduction:Acid dissociation constant can be defined as a quantitative measure of the strength of anacid in solution. It is the equilibrium constant of the dissociation reaction of an acid and issymbolized as ka. The dissociation constant is usually written in units of mol/L. In thisexperiment of determination of acid dissociation constant, a pH titration is performed todetermine the molecular mass of an unknown acid. According to the Bronsted-Lowery Theory,an acid is a substance that can donate a proton or a hydrogen ion, whereas a base is asubstance that can accept a proton. Thus, an acid-base reaction involves the transfer of a protonor protons from an acid to a base. The strength of an acid is measured by the extent to whichthe proton transfer occurs, and likewise the strength of a base is measured by the extent towhich the base accepts protons. In order to measure and compare acid strengths, the samebase is used in the transfer reaction, and the base usually is water.Acid strength is determined by titrating the acid with a base of known strength. Differentacids vary widely in acid strength, ranging from strong acids, which completely dissociate, andweak acids, which remain to great extent undissociated. Because of this variation in acidstrength, it is often possible to identify an acid by knowing its strength. Normally, acid strengthis determined by titrating the acid with a base of known strength. Ka will be greater if the extentof the proton transfer by the acid is greater. For hydrochloric acid, Ka is 108, and for acetic acid,Ka is 1.8 x 10-5. Hydrochloric acid is a c=strong acid while acetic is a weak acid. It is necessary to
find out the value of pH at the point where [A-] = [HA] to find Ka. pH stands for potential ofhydrogen and it is the measure of acidity or alkalinity of a solution.An equivalence point is indicated when there is a rapid change in pH with the addition ofa small quantity of base. Equivalence point is the stoichiometric point for the reaction of thebase with the acid. The acid is titrated as a diprotic acid if the resulting titration curve has twoequivalence points. Diprotic acid is when the two hydrogen ions are dissociated from the acid.When the number of equivalents of base in the titration is just one, then it is called monoproticacid. Which is why monoprotic acid have only one pKavalues and diprotic have two pKavalues.The molar mass of the acid is determined by the volume of base required to reach theequivalence point where the acid is completely neutralized by the base, along with thestoichiometry of the reaction. In this experiment, standard solution of sodium hydroxide is usedto titrate a solution containing an unidentified acid. The pH meter is used to monitor theprogress of the reaction between the acid and the base. The acid strength and the molar mass

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Term
Summer
Professor
Gao
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