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

Data for table 3 :

Flask S3

[SCN^-] = Molarity * vol in L = 2*10^-3 M * 0.004 L = 8*10^-6 moles

Fe(SCN)^2+ = 8*10^-6 moles

Absorbance = 2- log(%T) = 2-log(65.8) = 0.182

Flask S5 :

[SCN^-] = Molarity * vol in L = 2*10^-3 M * 0.008 L = 16*10^-6 moles

Fe(SCN)^2+ = 16*10^-6 moles

Absorbance = 2- log(%T) = 2-log(45.4) = 0.343

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