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posted by  Sidewinder on 2/24/2008 8:49:20 AM  |  status: Closed  

Desperate help needed for Lab Calculations.

Course Textbook Chapter Problem
General Chemistry Chemistry: The Central Science (10th) by Brown, Lemay, Bursten 16 N/A
Question Details:
Sigh, I'm working 18 hours this weekend and I completely forgot I have a lab report due. Please if anyone can help me, even with one calculation or anything, please post, or putting me on the right track to determining the correct calculations. Lab reports count for a good part of my grade, so I extremely need alot of help with this one!
 
Please if you need more information or the lab procedure that we used, just let me know, I'll type it all out if I have to.
 
Data
A) Observation: Change in color intensity of the -equilibrium mixture when more added.
     Became darker red solution.
     Equilibrium shifts to the right
 
     Observation: Change in color intensity of the -equilibrium mixture when more added.
     Became darker red solution.
     Equilibrium shifts to the right
 
      Observation: Change in color intensity of the -equilibrium mixture when more added.
     Became clear solution
     Equilibrium shifts to the left
 
 
Volume(mL)
Volume(mL)
Volume(mL)
Absorbance Readings
Test Tube
5.00*10^-2 M Fe(NO3)3
5.00 *10^-4 M NaNCS
H2O
 
 
 
1
5
1
4
0.199
 
 
2
5
2
3
0.33
 
 
3
5
3
2
0.726
 
 
4
5
4
1
0.915
 
 
5
5
5
0
1.06
 
 
Standard solution of "known concentration
 
0.207
 
 

Calculations
 
Experimental Method - Instrumental
 
Equilibrium concentrations and equilibrium constant
 
 
 
 
Molarity of FeNCS^2+ in solution of "known concentration" (M)
 = ? (M)
 
 
 
Test Tube
Initial Number of Moles
Total Volume (mL)
Number of moles at equilibrium
 
 
Fe^3+                  NCS-
 
Fe^3+
NCS-
FeNCS^2+
1
 
 
 
 
 
 
2
 
 
 
 
 
 
3
 
 
 
 
 
 
4
 
 
 
 
 
 
5
 
 
 
 
 
 
 
 
 
 
 
 
 
Test Tube
Equilibrium concentrations ( M ) 
 
Equilibrium constant, K 
 
Fe^3+ 
NCS-
FeNCS^2+
 
 
 
1
 
 
 
 
 
 
2
 
 
 
 
 
 
3
 
 
 
 
 
 
4
 
 
 
 
 
 
5
 
 
 
 
 
 
 
 
 
 
 
 
 

Post lab Questions -
 
1) For which test tube should your value for the equilibrium constant be most reliable?
Please explain your answer.
 
2) For which test tube should your value for the equilibrium constant be least reliable? Please explain your answer.
 
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AAnswers:

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Apprentice
Karma Points: 109
posted by koh on 2/24/2008 3:01:35 PM  |  status: Live
Asker's Rating: Lifesaver   
Sidewinder's comment:
"Thank you so much!"
Response Details:
You assume that all your NCS is converted to FeNCS because of the excess of Fe added.
Therefore. You calculate moles of NCS from your .0005M NaNCS solution, then divide this by the volume of your known concentration (20ml)

.0005M NaNCS = moles NCS / 0.002L = moles of  FeNCS   :   moles FeNCS/ 0.02L = M FeNCS


Equilibrium concentration of FeNCS

A1 / known Concentration = A2 / unknown concentration   (tubes 1-5)

Here you will solve for the equilibrium concentration of FeNCS of test tubes 1-5. you will put these answers in the bottom part of the calculations sheet.

A1 = absorbance of your “known”
Known concentration = what you solved for above
A2 = absorbance reading for each tube


Initial moles of Fe3+

Concentration of Fe(NO3)3 = moles / L    

You will use the concentration of the stock Fe(NO3)3, solution; for volume you will use the amount that you put into the test tubes (1-5)
In the case of Fe(NO3)3, you put 5ml or .005L into each test tube.

Initial moles of NCS

Concentration of NaNCS = moles / L

Same as above, concentration of the NaNCS solution was different. Also you put different amounts into each test tube 1-5.

Total volume of solution

10ml or 0.01L for each test tube

Volume of Fe(NO3)3 + Volume of NaNCS + Water





Moles of FeNCS at equilibrium

Concentration of FeNCS at equilibrium = moles / 0.01L

Use the concentrations that you solved for above (tubes 1-5). 0.01L comes from that each test tube had 10ml in it.


Moles of Fe3+ at equilibrium

Initial moles of Fe3+ - moles of FeNCS at equilibrium = moles of Fe3+ at equilibrium


Moles of NCS at equilibrium


Initial moles of NCS – moles of FeNCS at equilibrium = moles of NCS at equilibrium

It’s possible to get a negative # here, if your absorbance readings weren’t accurate.


Equilibrium concentration of Fe3+

Moles of Fe3+ at equilibrium / Volume of solution (0.01L) = Concentration of Fe3+ at equilibrium


Equilibrium concentration of NCS

Moles of NCS at equilibrium / Volume of solution (0.01L) = concentration of NCS at equilibrium


Equilibrium constant K


K = [FeNCS] / [Fe3+] [NCS]       Plug in your equilibrium concentrations here. Do this for test tubes 1-5.
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