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Characterization of a Weak Acid LabDaniel ParkAB1April 5, 20171
AbstractThe purpose of this lab was to calculate the molar mass and pKaof an unknown acid fromtitration with a standardized NaOH and then determine the weak acid. The weak acid was titratedtotal of six times, three from manual and three from using a LabQuest and a DropCounter. Themolar mass was determined to be 132.39 g/mol and the acid was determined to be monoproticwith a pKaof 2.92 therefore the unknown acid was determined to be salicylic.IntroductionThe purpose of this lab was to titrate an unknown weak acid with a standardized solutionof NaOH and find its titration curve, molar mass, Kaand thus pKa. Then with those values,identify the unknown acid. To achieve this objective titration technique with a Labquest andmanual titration was used.Titration is used to determine a concentration of an acid or base with a solution of knownconcentration of base or acid. In titration the volume of the solution of known concentration andpH are recorded. The pH is a numerical value representing the acidityof a solution and itindicates how much hydronium ion are in the solution (Equation 7). Plotting the pH on the y axisand the volume of the solution with known concentration on the x axis, the titration curve can begraphed. However, for some very weak acids that are hard to determine the equivalence pointgraphing the change of volume in the y axis and the change in the pH in the x axis may be moreeffective in finding the equivalence point4. The titration curve is useful in finding whether theacid is monoprotic or polyprotic. When an acid is monoprotic this means that the acid has onlyone hydrogen ion it can lose. On the other hand, polyprotic acids can lose multiple hydrogenions. However, the all of the acid molecules lose one hydrogen before losing additional hydrogenions therefore the titration curve of polyprotic acids have multiple equivalence points that2
correspond to the number of hydrogen ions it can lose. The equivalence point is where all theacid has completely lost one hydrogen ion from the compound. The polyprotic acids havemultiple acid ionization constants, Ka, as well while, monoprotic acids only have one (Equation9-10). The acid ionization constant is equilibrium constant for chemical reactions for a weak acidand it is unique to each acid at certain temperature. A large Kausually means a strong acid and asmaller Kameans the opposite since stronger acids dissociate completely. The acid ionizationconstant can be calculated using equation 9 and equation 10 for the second the acid ionizationconstant if the acid is polyprotic. The titration curve also is useful to find the equivalence pointand the halfway point. The halfway point is half the volume of NaOH needed for a completereaction or equivalence point. Using the halfway point the pKacan be found very easily. Since atthe halfway point the number of moles of OH-, base, equals the number of acid consumed, HA,