A student determining the equilibrium constant for the formation of the FeNCS 2+

Question

A student determining the equilibrium constant for the formation of the FeNCS 2+ ion followed the procedure in this experiment: in Part I, 2.00x10-1M Fe3+ and 2.00x10-3M SCN- ion stock solutions were used. All standard solutions were prepared in 100-mL volumetric flasks. Complete the following calculations: 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.

Explanation / Answer

Solution :-

We can use the dilution formula to calculate the concentration of the each species

For the S2 the transmitanve is 80.2 for the 4.00*10^-5 M solution

Calculating the concentration in the column S3

4.00 ml of the NaSCN diluted to 100 ml

So

[SCN-] = 65.8 * 4.00*10^-5 M / 80.2 = 3.25*10^-5 M

[FeSCN^2+] = 3.25*10^-5 M

Absorbance A = 2 – log % T

                           = 2 – log 65.8

                           = 0.1818

Calculating for the coloumn S5

[SCN-] = 45.4 * 4.00*10^-5 / 80.2 = 2.26*10^-4 M

[FeSCN^2+] = 2.26*10^-4 M

Absorbance A = 2 – log % T

                           = 2 – log 45.4

                           = 0.3429

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