% Ammonium Ion in an Unknown Salt By Back Titration Experiment 8 % Ammonium Ion in an Unknown Salt By Back Titration

Goals Make standardized solution of ______ Determine _________ in unknown Overall: Determine ______________ ____________________ Possibilities: _____,_______,_______,______

Hazards HCl NaOH Unknowns Methyl Red

Standardized HCl The reaction How do you determine the endpoint? previously standardized NaOH is titrated with your HCl with unknown molarity HCl + NaOH  Na+ + Cl- + H2O How 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 ______

Molarity? Known: 25.00mL NaOH was titrated (use pipet) M (mol/L) NaOH, previously determined Volume of HCl delivered (convert mL to L) Reaction is 1:1

An Analogy: Kids in a Candy Store* Give child $1.00 Send into store to buy candy bar Brings back 35¢ How much was the candy bar? $1.00 - 35¢ = 65¢ Indirect Assumptions: Honest child brings back all change Bought only one candy bar *Adapted from Last, A.M. J. Chem. Educ. 1998, 75, 1121.

Applying to Titration Start with unknown quantity of acid Add 1.00 mol of monoprotic base Titrate acid remaining to reveal that there is 0.35 moles left over How many moles of acid were present? How many moles of base reacted 1.00mol – 0.35mol = 0.65mol 1:1 tells us that there were 0.65moles acid Assumptions: There was an excess of base added Acid reacted only with base in irreversible manner 1:1 reaction ratio

Back Titration Why can’t we do direct acid-base titration? Weigh unknown Add 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 interest NH4OH  NH3 + H2O, boil off NH3 and no NH4OH to react previous reaction back to the left Use litmus to make sure all of NH3 is gone Titrate leftover excess substance OH-(from base) + H+(HCl)  H2O

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 NaOH 1mol NaOH:1mol NH4+ Molar mass NH4+ (g/mol)

Critical Measurements

Introduction What is being determined Why doing back-titration What is a back-titration Explain the reactions that are occurring Tell why a particular step is done Tell what is reacting with what to produce what Tell why particular things are added: What do they accomplish What indicator is used and why What is the condition of the system at the end

Experimental Design Variables