Analytical Chem istry - DePauw University

Chapter 9 Titrimetric Methods451determination of a compound’s equivalent weight and its acid or its basedissociation constant.Eq u i v a l e nt We i g h t sSuppose we titrate a sample containing an impure weak acid to a welldefinedend point using a monoprotic strong base as the titrant. If we assumethat the titration involves the transfer of n protons, then the moles oftitrant needed to reach the end point isn molestitrantmolestitrant = × molesanalytemolesanalyteIf we know the analyte’s identity, we can use this equation to determine theamount of analyte in the sample1moleanalytegramsanalyte = molestitrant× × FW analyten molestitrantwhere FW is the analyte’s formula weight.But what if we do not know the analyte’s identify? If we can titrate apure sample of the analyte, we can obtain some useful information that mayhelp in establishing its identity. Because we do not know the number ofprotons being titrated, we let n = 1 and replace the analyte’s formula weightwith its equivalent weight (EW)1equivalentanalytegramsanalyte = molestitrant××EW analyte1moles titrantwhereFW= n×EWExample 9.6A 0.2521-g sample of an unknown weak acid is titrated with 0.1005 MNaOH, requiring 42.68 mL to reach the phenolphthalein end point. Determinethe compound’s equivalent weight. Which of the following compoundsis most likely to be the unknown weak acid?So l u t i o nascorbic acid C 8 H 8 O 6 FW = 176.1 monoproticmalonic acid C 3 H 4 O 4 FW = 104.1 diproticsuccinic acid C 4 H 6 O 4 FW = 118.1 diproticcitric acid C 6 H 8 O 7 FW = 192.1 triproticThe moles of NaOH needed to reach the end point is