Department of Chemistry and Geosciences

Honors 1084 Lab Spring 2005

Lab Test 4

Name:_________________________________________________ Section:

Answer the following questions to the best of your ability showing all work necessary to obtain the answer. The answer must be given with the correct units and the correct number of significant figures where applicable. Remember to show all your work necessary to obtain the answer. Simply writing the correct answer without showing any work may not receive full credit.

1. (80 points) A solution was prepared by mixing 7.00 mL of 0.0030 M Fe³⁺, 9.00 mL of 0.0030 M HSCN, and 4.00 mL of 0.25 M HNO₃. Both the iron(III) ion solution and the HSCN solution were prepared with 0.25 M HNO₃. The absorbance of the solution was measured at 0.754.

(a) (30 points) Calculate the concentrations of Fe³⁺(aq) and HSCN immediately after mixing before the formation of the iron thiocyanate complex. (These are the initial concentrations).

Using the dilution formula:

(b) (20 points) Given the absorptivity, e, for the iron thiocyanate complex is equal to 4950 L mol^-1 cm^-1, calculate the equilibrium concentration of the iron thiocyanate complex using Beer’s Law. (Beer’s Law: A = elc; where “l” = 1.00 cm, “A” = absorbance, and “c” = molar concentration).

Solving Beer’s Law for concentration:

(c) (30 points) Using the equilibrium concentration of the iron thiocyanate complex, calculate the equilibrium concentrations of Fe³⁺ and HSCN, as well as the equilibrium constant for the reaction:

Fe³⁺(aq) + HSCN(aq) Fe(SCN)²⁺(aq) + H⁺(aq)

The equilibrium concentrations of Fe³⁺ and HSCN are equal to the initial concentrations minus the concentration of FeSCN²⁺:

Equilibrium concentration of Fe³⁺ = 0.0011 M – 0.00015 M = 0.00095 M

Equilibrium concentration of HSCN = 0.0014 M – 0.00015 M = 0.00125 M

The equilibrium constant is equal to:

2. (20 points) Write the ionization reaction and the equilibrium expression associated with the acid ionization constant, K_a.

For a generic acid, HA:

The equilibrium expression for this reaction is: