Question

1) Calculate the initial concentrations for all the solutions .

2) explain why we use excess of Fe(NO3)3 compared to KSCN in solution #1

HNO3 0,50 M (mL) Table 1: Volumes of solutions for each experiment. Fe(NO3)3 Fe(NO3)3 KSCN Solution # 0,200 M 2,00 x 10-3 M 2,00 x 10-3 M (mL) (mL) (mL) 5 0 1 0 5 5 0 5 4 0 so 5 3 0 5 2 0 5 uuu

Answer 1

Answer:

The total volume = 10 mL   for all the solutions 1 to 6

use the formula V1C1 = V2C2 to calculate the final concentration in the solution.

Solution 1:

Volume and concentration of Fe(NO3)3 = 5 mL of 0.200 M

               final volume is 10 mL

                                             C2 = V1C1 / V2 = 5 x 0.2 / 10 = 0.1 M

Volume and concentraion of KSCN = 1 mL of 2 x 10-3 M

             final volume is 10 mL

                                          C2 = V1C1 / V2 = 1 x 2 x 10-3 / 10

                                               = 2 x 10-4 M

Volume and concentration of HNO3 = 4 mL of 0.5 M

           final volume is 10 mL

                                       C2 = V1C1 / V2 = 4 x 0.5 / 10

                                                             = 0.2 M

Thus the solution 1 has initial concentrations of

   Fe(NO3)3 = 0.1 M

                                         KSCN = 2 x 10-4 M

                                      HNO3 = 0.2 M

Similarly for solution 2

Fe(NO3)3 = 5 x 2 x 10-3 / 10 = 1 x 10-3 M

KSCN     = 5 x 2 x 10-3/10 = 1 x 10-3 M

HNO3    =   0 M

for solution 3

Fe(NO3)3 = 5 x 2 x 10-3 / 10 = 1 x 10-3 M

KSCN     = 4 x 2 x 10-3/10 = 8 x 10-4 M

HNO3    =   1 x 0.5 / 10 = 0.05 M

for solution 4

Fe(NO3)3 = 5 x 2 x 10-3 / 10 = 1 x 10-3 M

KSCN     = 3 x 2 x 10-3/10 = 6 x 10-4 M

HNO3    =   2 x 0.5 / 10 = 0.10 M

for solution 5

Fe(NO3)3 = 5 x 2 x 10-3 / 10 = 1 x 10-3 M

KSCN     = 2 x 2 x 10-3/10 = 4 x 10-4 M

HNO3    =   3 x 0.5 / 10 = 0.15 M

for solution 6

Fe(NO3)3 = 5 x 2 x 10-3 / 10 = 1 x 10-3 M

KSCN     = 1 x 2 x 10-3/10 = 2 x 10-4 M

HNO3    = 4 x 0.5 / 10 = 0.20 M

2)

Consider the reaction Fe3+ + SCN- $\rightleftharpoons$ Fe(SCN)2+

In solution 1 , the concentration of Fe3+ is 0.1 M

                    the concentration of SCN- is 2 x 10-4 M

[Fe3+] / [SCN-] = 0.1 / 2x 10-4 = 500

That is the concentration of Fe3+ is 500 times more than that of SCN-

According to lechatlier principle this will shift the reaction more to the product side and this ensure all the SCN- ions are completely converted to product.

Thus the solution 1 will act as the standard for the thiocyanate complex for 100% conversion.

This will help to study the other sample's conversion from the measurement of product by absorbance.