3. Use the mean value you determined for Keq to calculate the SCN- concentration

Question

3. Use the mean value you determined for Keq to calculate the SCN- concentration in a solution whose initial Fe^3+ Concentration was 4.00 x 10^-2 M and initial SCN concentration was 1.00 x 10^-3 M. Is all of the SCN- in the form of FeNCS^2+? Calorimetric Determination of an Equilibrium Constant in Aqueous Solution 4. Does the result of the calculation in question 3 justify your original assumption that all of the SCN- is in the form of FeNCS^2+ 5. based upon your answer to question 4, is the measured value of Keq too high or too low? keq is 3279.63

Explanation / Answer

The reaction will be

Fe 3+ + SCN- --> FeSCN 2+

.......                 Fe 3+ .......SCN-.........FeSCN 2+
Initial.                0.04      0.001             0
C                         -x           -x                 x
Equilibrium        0.04-x     0.001-x.       x

Kequ = 3279.63 = x / (0.04-x)(0.001-x)
3279.63 = x / (x^2 - 0.041x + 4x10^-5)
3279.63 x^2 - 134.46x + 0.131 = x
3279.63 x^2 - 133.46x + 0.131= 0
x = 0.03968M ( not possible) or 0.0010M

So whole amount of SCN- will be consumed

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