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

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

3. Hazards
HCl
NaOH
Unknowns
Methyl Red

4. 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 ______

5. 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

6. 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?
$ ¢ = 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.

7. 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

8. 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

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

10. Critical Measurements

11. 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

12. Experimental Design Variables