Question

# Enter your answer in the provided box. In a dilute nitric acid solution, Fe3+ reacts with...

 Enter your answer in the provided box. In a dilute nitric acid solution, Fe3+ reacts with thiocyanate ion (SCN−) to form a dark red complex: [Fe(H2O)6]3+ + SCN− ⇌ H2O + [Fe(H2O)5NCS]2+ The equilibrium concentration of [Fe(H2O)5NCS]2+ may be determined by how dark the colored solution is (measured by a spectrometer). In one such experiment, 1.0 mL of 0.20 M Fe(NO3)3 was mixed with 1.0 mL of 3.8 × 10−3M KSCN and 8.0 mL of dilute HNO3. The color of the solution quantitively indicated that the [Fe(H2O)5NCS]2+ concentration was 7.3 × 10−5M. Calculate the formation constant for [Fe(H2O)5NCS]2+.

Initial Molarity of Fe(NO3)3 (M1)= 0.20 M

Volume (V1)= 1.0mL

After mixing, total Volume (V2)= 1mL + 1mL + 8.0 mL = 10.0 mL

Let Molarity of Fe(NO3)3 in final solution be M2

Using Molarity equation, we have :

M1V1 = M2V2

0.20 M * 1.0 mL = M2 * 10mL

M2 = 0.020 M

Similarly Molarity of KSCN in final solution = 1.0 mL * 3.8 *10-3 M / 10 mL = 3.8 * 10-4 M

Reaction is:

[Fe(H2O)6 ]3+ + SCN- H2O + [Fe(H2O)5NCS]2+

I(M) 0.02 3.8 * 10-4 0

C(M) -7.3 * 10-5 -7.3 * 10-5​ +7.3 * 10-5

E(M) 0.0199 0.000307 +7.3 * 10-5

So, Kf = [Fe(H2O)5NCS]2+ / (   [Fe(H2O)6 ]3+ [SCN- ]

Kf = (7.3 * 10-5​ ) / (0.000307)(0.0199) = 11.94 12

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