Question

of ree lu- Method of Continuous Variation 19-15. Method of continuous variation. Make a graph of absor- bance versus mole fraction of thiocyanate from the data in the table. ble. aty mL Fe and nce 6.00 mL SCN Absorbance solution solution at 455 nm 30.00 0.001 27.00 3.00 0.122 24.00 0.226 21.00 9.00 0.293 18.00 12.00 0.331 15.00 15.00 0.346 12.00 18.00 0.327 9.00 21.00 0.286 6.00 24.00 0.214 3.00 27.00 0.109 0 30.00 0.002 a. Fel solution: 1.00 mM Fe(NO3), + 10.0 mM HNO, b. SCN solution: 1.00 mM KSCN + 15.0 mM HCI. SOURCE: Z D. Hill and P. MacCarthy, "Novel Approach to Job's Method." J. Chem. Ed. 1986, 63, 162. (a) What is the stoichiometry of the predominant Fe(SCN)- species? (b) Why is the peak not as sharp as those in Figure 19-10? (e) Why does one solution contain 10.0 mM acid and the other 15.0 mM acid? thm em. er's cher is a

Answer 1

a) The stoichometry is determined by the maximum point in the plot, this value corresponds to the absorbance value of 0.346, at this point, there were added equivalent volumes of Fe³⁺ and SCN^- solution so, the stoichiometry is 1:1, that means Fe(SCN)²⁺is the predominant species.

b) In this case the peak is not sharp due to the stability of the complex that is formed, Fe(SCN)²⁺ is a weak complex.

c) The concentration of the acids vary to maintain a constant ionic strength,a requirement of this method.