Calculations

Graphing the relationship between absorbance and [FeSCN²⁺]

Assuming that “all” of the SCN^– ions have been converted to FeSCN²⁺ ions, calculate [FeSCN²⁺] in each of the solutions in Part 1. For example, in test tube 1-2, 1.0 mL of a 2.00 χ 10^–3 M KSCN solution was diluted to 10.0 mL. The concentration of SCN^– that results from this dilution is the one to use for determining [FeSCN²⁺]. Because of the 1:1 stoichiometry, that initial concentration of SCN^– is equal to [FeSCN²⁺].

Plot a full-page graph of the absorbance against the concentration of FeSCN²⁺ in all standard solutions. Use a ruler to draw the best straight line that comes closest to each of your five data points. Your line should pass through (0 M, 0). (Why?) This graph is your calibration curve. When you measure the absorbance of a solution that contains an unknown concentration of FeSCN²⁺ ions, you can use this calibration curve to determine the unknown concentration.

REMEMBER - SHOW AN EXAMPLE OF THESE CALCULATIONS IN YOUR CALCULATIONS SECTION

Part 2: Determining the equilibrium constant

Calculate the value of the equilibrium constant for Reaction (1) from your data for each of the five solutions in Part 2. You may find it helpful to organize each of your five calculations in an equilibrium table such as this one:

The calculations for Part 2 are most easily done by following Steps 1 through 5, which are illustrated in the sample calculations below.

Step 1. Find the initial concentrations of Fe³⁺ and SCN^– in the mixtures in test tubes 2-1 through 2-5. You will need to account for dilutions of the stock solutions.

Step 2. Use the measured absorbances of these solutions and your calibration curve to find the equilibrium concentration of [FeSCN²⁺] in these solutions.

Step 3. Find the change in concentration of FeSCN²⁺. Deduce the changes in concentration of the reactants, Fe³⁺ and SCN^–. The changes in concentration of all three species should be consistent with the stoichiometry of the reaction.

Step 4. Find the equilibrium concentrations of the reactants, Fe³⁺ and SCN^–.

Step 5. Calculate K_c for the reaction for each of the mixtures by substituting values for the equilibrium concentrations of Fe3+, SCN–, and FeSCN2+ into the equilibrium constant expression.