4 Standardized HCl The reaction How do you determine the endpoint? previously standardized NaOH is titrated with your HCl with unknown molarityHCl + NaOH Na+ + Cl- + H2OHow do you determine the endpoint?Look at reaction:Start __________End __________Determine Indicator_______________________________ in basic (> 6.0)_________ in acidic, change occurs in the 3-6 pH range___________ in between, must go all the way to ______
5 Molarity? Known: 25.00mL NaOH was titrated (use pipet) M (mol/L) NaOH, previously determinedVolume of HCl delivered (convert mL to L)Reaction is 1:1
6 An Analogy: Kids in a Candy Store* Give child $1.00Send into store to buy candy barBrings back 35¢How much was the candy bar?$ ¢ = 65¢IndirectAssumptions:Honest child brings back all changeBought only one candy bar*Adapted from Last, A.M. J. Chem. Educ. 1998, 75, 1121.
7 Applying to Titration Start with unknown quantity of acid Add 1.00 mol of monoprotic baseTitrate acid remaining to reveal that there is 0.35 moles left overHow many moles of acid were present?How many moles of base reacted1.00mol – 0.35mol = 0.65mol1:1 tells us that there were 0.65moles acidAssumptions:There was an excess of base addedAcid reacted only with base in irreversible manner1:1 reaction ratio
8 Back Titration Why can’t we do direct acid-base titration? Weigh unknownAdd a reagent that reacts with substance of interest in excess (25.00mL)NH4+(limit.reag.)+OH-(excess) NH4OH + OH-(from base)Ensure equilibrium cannot shift back to substance of interestNH4OH NH3 + H2O, boil off NH3 and no NH4OH to react previous reaction back to the leftUse litmus to make sure all of NH3 is goneTitrate leftover excess substanceOH-(from base) + H+(HCl) H2O
9 Data and Results Known Mass unknown (g) MNaOH (mol/L) Volume NaOH used (25.00mL => 0.025L)MHCl (mol/L)Volume HCl delivered (mL => L)1mol HCl:1mol NaOH1mol NaOH:1mol NH4+Molar mass NH4+ (g/mol)
11 Introduction What is being determined Why doing back-titration What is a back-titrationExplain the reactions that are occurringTell why a particular step is doneTell what is reacting with what to produce whatTell why particular things are added: What do they accomplishWhat indicator is used and whyWhat is the condition of the system at the end