To obtain [FeSCN 2+ ] eq in tube 5, you made the assumption that 100% of the ion

Question

To obtain [FeSCN2+]eq in tube 5, you made the assumption that 100% of the ions SCN– had reacted. Now that you know the value for Kc ((208.385), build an ICE diagram to calculate the true percentage of SCN– reacted in tube 5.
Tube 5 is the one with .888 Au
How do I find the true percentage of SCN-?

Absorbance             Initial concentrations Concentrations at equilibrium Kc* [Fe3+]o [SCN-]0 [FeSCN2+]eq [Fe3+]eq [SCN-]eq (au) (M) (M) (M) (M) (M) (M-1) .888 0.200 .00200 0.00200 .250 0.001 0.0004 0.0000600 0.000940 0.00034 187.5 .420 0.001 0.0006 0.000100 0.000900 0.000500 222.2 .520 0.001 0.0008 0.000124 0.000876 0.000676 209.46 .640 0.001 0.001 0.0001535 0.0008465 0.0008465 214.38 Average 208.385

Explanation / Answer

First of all we need to now what is our equation.

Fe3+(aq)+SCN–(aq)FeSCN2+(aq) equation (1)

The equilibrium constant is given by

Kc=[FeSCN2+]eq/[Fe3+]eq[SCN-]eq

Now our ICE table, as we want to recalculate the initial values of the SCN- we assume that initial concentracions are our X.

Now we calculate the Value of X from the Kc equation

   208.385 = 0.00200 / (X-0.00200)2

(X-0.00200)2 = 0.00200/208.385

(X-0.00200)2 = 9.6x10-6

X2 - 0.00200X +(0.002)2 = 9.6x10-6

X2 - 0.00200X +4x10-6 -9.6x10-6 = 0

X2 - 0.00200X -5.6x10-6 = 0

now we solve our cuadratic equation to find X

We find X1 = 0.0035 and X2= -0.0015 we can´t have a negative concentration so X1 is the best choice.

Now we have that the [SCN-] initial is 0.0035 M.

To determine how much did react we substract the 0.00200 of the FeSCN2+ formed.

Then the quantity of SCN- in eq is = 0.0035-0.002 = 0.0015 M (this is the [SCN-]eq)

to find the percentage reacted we divide the [ ] eq / [ ]o x100

0.0015 / 0.0035 x100 = 42.85 % of SCN- did react.

Hope this works for you!

Fe3+ (M) SCN- (M) FeSCN2+ (M) Initial X X 0 Change -0.00200 -0.00200 0.00200 Equilibrium X-0.00200 X-0.00200 0.00200

Related Questions

Navigate

• Browse (All)
• Browse T
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.