Find the equilibrium constant.

1. The equilibrium constant for the reaction,!
Fe3+ + SCN- ⇋ FeSCN2+ (at a specific temperature),
can be determined by first preparing a standard solution of FeSCN2+ and comparing its absorbance of light to an equilibrium system of unknown concentrations.
i.) Standard preparation:
25.0 mL of 0.500 M Fe(NO3)3 + 5.00 mL of 0.00500 M KSCN, Absorbance (A) = 0.700
☝ (excess!)
Calculate the equilibrium concentration of FeSCN2+ for this standard:
ii.) Unknown equilibrium system:
30.0 mL of 0.00400 M Fe(NO3)3 + 20.0 mL of 0.00400 M KSCN, Absorbance (A) = 0.400

******My answer: 221

2.) ! !
A student investigated the same equilibrium system as above, but at a new temperature, by first preparing a standard solution:
36.0 mL of 0.300 M Fe(NO3)3 + 4.00 mL of 0.00300 M KSCN, !Astd = 0.525
And then an equilibrium system of unknown concentrations:
25.0 mL of 0.00300 M Fe(NO3)3 + 15.0 mL of 0.00300 M KSCN ; ! A = 0.442
! Calculate the equilibrium constant for this new temperature (to 3 sig figs).

* Ignore the exclamation points.
****My answer: 179

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