Experiment 21 Name Section Data: Determination of the Equilibrium Constant for a

Question

Experiment 21 Name Section Data: Determination of the Equilibrium Constant for a chemical Reaction Method Valuune in ml, Volume in mla 2.00 x 10 N 2.00 x 10 Volume in ml. Method Fe (NO)a of Water AMLrnure 1.00 4.00 5.00 x 10-4 M 2.00 3.00 x 10- M 3.00 x 10" M x 104 M 1.00 4.00 5.00 to x 10-4 M 0.00 5.00 5.00 If Method II was used, [FescN2 x 10-4 M: FescNa lin Mixtures l to 5 found by Equation T. Calculations A. Calculation of Ke assuming the reaction This calculation is most easily done by following Steps l through 5 in the discussion. Results are to be entered in the table on the following page. two Step 1. Find the initial number of moles of Fe and SCN- in the mixtures in test tubes l through 5. Use Equation 3 and enter the values in the first columns of the table. each of the mixtures in the next to last column in the table. Use Step 2. Enter the experimentally determined value of [FescNa at equilibrium for the table. Note that this is Equation 3 to find number of moles of FescN2+ in each ofthe mixtures, and enter the values in the fifth column of following page) also the number the that were used up in the reaction. (conalued on of moles of Fe3 and SCN-

Explanation / Answer

Calculations for equilibrium constant

[Fe3+]initial

Sample 1-5 : [Fe3+]initial = 2 x 10^-3 M x 5 ml/10 ml = 1 x 10^-3 M

[SCN-]initial

Sample 1 : [SCN-]initial = 2 x 10^-3 M x 1 ml/10 ml = 2 x 10^-4 M

Sample 2 : [SCN-]initial = 2 x 10^-3 M x 2 ml/10 ml = 4 x 10^-4 M

Sample 3 : [SCN-]initial = 2 x 10^-3 M x 3 ml/10 ml = 6 x 10^-4 M

Sample 4 : [SCN-]initial = 2 x 10^-3 M x 4 ml/10 ml = 8 x 10^-4 M

Sample 5 : [SCN-]initial = 2 x 10^-3 M x 5 ml/10 ml = 1 x 10^-3 M

molar absorptivity of [FeSCN]2+ is required here to calculate [FeSCN]2+eq value

say, 7000 M-1.cm-1 is molar absorptivity

then,

[FeSCN]2+eq = absorbance/molar absorptivity

For Sample 1,

[FeSCN]2+eq = 0.07/7000 = 1 x 10^-5 M

So,

equilibrium concentration of

[Fe3+]eq = [Fe3+]initial - [FeSCN]2+eq = 1 x 10^-3 - 1 x 10^-5 = 9.9 x 10^-4

[SCN-]eq = [SCN-]initial - [FeSCN]2+eq = 2 x 10^-4 - 1 x 10^-5 = 1.9 x 10^-4 M

Equilibrium constant Keq,

Keq = [FeSCN]2+eq/([Fe3+]eq . [SCN-]eq)

       = 1 x 10^-5/(9.9 x 10^-4)(1.9 x 10^4)

       = 53.16

Similarly other solutions can be calculated.

[Check molar absorptivity given for your experiment]

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