A student prepared a flask at some non standard temperature. she found the absor
ID: 793890 • Letter: A
Question
A student prepared a flask at some non standard temperature. she found the absorbance of her solution and then determined that the eqilibrium concentration of FeSCN^2+ was 0.00031M.
the flask had: 3mL of 0.002M Fe(NO3)3, 3mL of 0.002M KSCN, 4mL of H20.
a) determine the initial concentrations of voth Fe^3+ and SCN- in the solution that would exist if no reaction happened.
b)determine the equilibrium concentrations of Fe^3+ and SCN- using the results from part a and the fact that the equilibrium concentration of FeSCN^2+ is known.
c) determine the equilibrium contant.
prepare an ICE chart so i can try to understand the process, thanks!
Explanation / Answer
(a) Moles of Fe3+ = volume x concentration of Fe(NO3)3
= 3/1000 x 0.002 = 6 x 10^(-6) mol
Moles of SCN- = volume x concentration of KSCN
= 3/1000 x 0.002 = 6 x 10^(-6) mol
Total volume = 3 + 3 + 4 = 10 mL = 0.01 L
Initial concentration of Fe3+ = moles of Fe3+/total volume
= 6 x 10^(-6)/0.01 = 0.0006 M
Initial concentration of SCN- = moles of SCN-/total volume
= 6 x 10^(-6)/0.01 = 0.0006 M
(b)...Fe3+...+...SCN-...<=>...Fe(SCN)2+
I....0.0006......0.0006...................0
C......-a.............-a.....................+a
E..0.0006-a.....0.0006-a.......... ...a
At equilibrium:
[Fe(SCN)2+] = a = 0.00031 M
[Fe3+] = 0.0006 - a = 0.00029 M
[SCN-] = 0.0006 - a = 0.00029 M
(c) Equilibrium constant K = [Fe(SCN)2+]/[Fe3+][SCN-]
= 0.00031/(0.00029 x 0.00029)
= 3686 = 3.7 x 10^3
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