Number 3 In preparing solution number 1 (see experimental procedure) a student m

Question

Number 3 In preparing solution number 1 (see experimental procedure) a student measure 5.0 mLof2.00 times 10^2 M Fe(NO_3)_2, 2.0 mL of 2.00 times 10^3 M HSCN, and 3.0 mL of 0.50 M HNO_3. The solvent for the Fe(NO_3), and HSCN solutions is 0.50 M HNO_3). After combining these, what are the concentrations of Fe^3+ SSCN. and H^+ in the resulting solution? (These are the initial concentrations for prelab question #3), When at equilibrium, the solution prepared in question #1 has an absorbance of 0.36. What is the Fe(SCN)^2+ concentration, assuming that the absorptivity of Fe (SCN)^2+ 5150 L mol^3+ cm^-1 for 447-nm light Use the answers from question #1 as your initial concentrations and your answer from question #2 as the equilibrium concentration of Fe(SCN)^2+ (x), and begin filling in the equilibrium table (ICE table). Then determine the equilibrium concentrations of Fe^3+ HSCN, and H^+ Finally, determine the equilibrium constant for the system.

Explanation / Answer

(1) Intial moles of Fe3+ =2*10-3 M*0.005 lit=1*10-5 moles

Initial moles of HSCN=2*10-3 M*0.002 lit=4*10-6 moles

Total volume =10 ml

So [Fe3+]=1*10-5/0.01=0.001 M

[HSCN]=4*10-6/0.01=4*10-4 M

(2)

Eqm [FeSCN2+]=6.99*10-6 M given

[H+]=0.5 M constant (solvent HNO3)

So

Fe3++ HSCN -------> FeSCN2++ H+

Initial 0.001 0.0004 0 0.5

Change -6.99*10-6 -6.99*10-6   6.99*10-6  

equilibrium 9.93*10-4 3.93*10-4 6.99*10-6      0.5

So Kc=6.99*10-6*0.5/( 9.93*10-4 *3.93*10-4 )=8.95

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