4. A student determining the equilibrium constant for the formation of the FeNCS

Question

4. A student determining the equilibrium constant for the formation of the FeNCS2+ ion followed the procedure in this experiment. In Part L 2.00 x 10-1M Fes and 2.00 x 10-3M SCN ion stock solutions were used. All standard solutions were prepared in 100-mL volumetric flasks. Complete the following calculations, and enter the data in Table 3 on the next page (1) Calculate the initial molar SCN ion concentration in standard solutions S3 and S5 (2) Calculate the molar FeNCS2+ ion concentration in standard solutions S3 and S5 (3) Calculate the absorbances for standard solutions S3 and S5 (4) Prepare a Beer's Law plot, on the graph paper earlier in this experiment, for the FeNCS ion using data for standard solutions S1-S6 In Part II, 2.00 x 10-3M Fe ion and 2.00 x 10-3M SCN ion stock solutions were used to prepare the equilibrium mixtures indicated in Table 4 on the next page. The %T of the equilibrium solutions (E2-E4) are listed in Table 5 on the next page. Make the following calculations in the spaces provided, and enter the results in Table 5 (5) Calculate the initial Fest ion concentration in equilibrium mixtures E2-E4. (6) Calculate the initial SCN ion concentration in equilibrium mixtures E2-E4. (7) Express the %T readings for solutions E2-E4 as equivalent absorbances. (8) Use your Beer's Law plot to determine the equilibrium FeNCs2 ion concentration in solutions E2-E4. (9) Determine the equilibrium molar Fes+ ion concentration in solutions E2-E4. 10) Determine the equilibrium molar SCN ion concentration in solutions E2-E4. Calculate Keq for the formation of FeNCs ion at the experimental temperature, using the data for solutions E2-EA, (12) Use your calculated Keqs from (1) to calculate the mean Ke for the formation of FeNCs? ion at the experimental temperature

Explanation / Answer

Calculation of initial and final concentrations

Fe3+ + SCN- <==> FeNCS2+

Table 1 : Standard solutions

Solution                     initial Fe3+ (M)                                                  initial SCN- (M)

    S1              0.002 M x 10 ml/50 ml = 0.0004 M                   0.002 M x 0 ml/50 ml = 0.0 M

    S2              0.002 M x 10 ml/50 ml = 0.0004 M                0.002 M x 1 ml/50 ml = 0.00004 M

    S3              0.002 M x 10 ml/50 ml = 0.0004 M                0.002 M x 2 ml/50 ml = 0.00008 M

    S4              0.002 M x 10 ml/50 ml = 0.0004 M                0.002 M x 3 ml/50 ml = 0.00012 M

    S5              0.002 M x 10 ml/50 ml = 0.0004 M                0.002 M x 4 ml/50 ml = 0.00016 M

    S6              0.002 M x 10 ml/50 ml = 0.0004 M                0.002 M x 5 ml/50 ml = 0.00020 M

Here, initial [SCN-] = FeNCS2+ formed in solution

find absorbance using formula,

absorbance = 2 - log(%transmittance)

Plot absorbance vs FeNCS2+

Slope = molar absorptivity

Table 2 : Equilibrium mixture of FeNCS2+ ion study

Solution                     initial Fe3+ (M)                                                initial SCN- (M)

    E1              0.002 M x 5 ml/10 ml = 0.0002 M                   0.002 M x 0 ml/10 ml = 0.0 M

    E2              0.002 M x 5 ml/10 ml = 0.0002 M                0.002 M x 1 ml/10 ml = 0.0002 M

    E3              0.002 M x 5 ml/10 ml = 0.0002 M                0.002 M x 2 ml/10 ml = 0.0004 M

    E4              0.002 M x 5 ml/10 ml = 0.0002 M                0.002 M x 3 ml/10 ml = 0.0006 M

    E5              0.002 M x 5 ml/10 ml = 0.0002 M                0.002 M x 4 ml/10 ml = 0.0008 M

    E6              0.002 M x 5 ml/10 ml = 0.0002 M                0.002 M x 5 ml/10 ml = 0.0010 M

From absorbance values of E1 to E6, caluculate equilibrium concentration of FeNCS2+ present in solution by using formula,

concentration of FeNCS2+ = absorbance/molar absorptivity

Subtract concentratuon of FeNCs2+ from initial Fe3+ and initial SCN- concentration E1-E6 in Table 2 to get equilibrium concentrations of Fe3+ and SCN- in solution.

Calculate equilibrium constant

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