# 6.P: Determination of Kc for a Complex Ion Formation (Pre-Lab)


A student mixes 5.0 mL of 0.00200 M $$\ce{Fe(NO3)3}$$ with 5.0 mL 0.00200 M $$\ce{KSCN}$$. She finds that the concentration of $$\ce{FeSCN^{2+}}$$ in the equilibrium mixture is 0.000125 M. Follow these steps to determine the corresponding experimental value of $$K_{c}$$ for the reaction of $$\ce{Fe^{3+}}$$ and $$\ce{SCN^{-}}$$ to produce this complex ion. Show your calculations for each step below and then place the appropriate value(s) in the equilibrium (or 'ICE') table near the bottom of the page.

• Step 1. Calculate the molarity of $$\ce{Fe^{3+}}$$, $$\ce{SCN^{-}}$$, and $$\ce{FeSCN^{2+}}$$ initially present after mixing the two solutions, but prior to any reaction taking place. ($$M_{1}V_{1} = M_{2}V_{2}$$)
• Step 2. Determine the expression and initial value for $$Q_{c}$$. Then give the appropriate signs of the concentration changes for each species in terms of the reaction's shift, or $$x$$, into the 'ICE' table.
• Step 3. Fill in the equilibrium value for the molarity of $$\ce{FeSCN^{2+}}$$. From this, you can determine the value of $$x$$.
• Step 4. Given the value of $$x$$, determine the equilibrium molarities of $$\ce{Fe^{3+}}$$ and $$\ce{SCN^{-}}$$.
'ICE' Table
$$\ce{Fe^{3+}}$$ (aq) $$+ \quad \ce{SCN^{-} (aq)}$$ $$\ce{ <=>\quad FeSCN^{2+} (aq)}$$
I
C
E
• Step 5. Give the correct expression for $$K_{c}$$ for this equation. Then calculate the value of $$K_{c}$$ for the reaction from the equilibrium concentrations. Use correct significant figures.
• Step 6. On the reverse side, complete an 'ICE' table using this same procedure, but using a different reaction stoichiometry: $$\ce{Fe^{3+} + 2 SCN^{-} <=> Fe(SCN)2^{2+}}$$ (ignore that fact that this is incorrect since the reaction is not balanced with respect to charge). Assume that the equilibrium concentration of $$\ce{FeSCN^{2+}}$$ is 0.0000625 M, or one-half its previous value. Remember how the reaction stoichiometry affects the expression for $$K_{c}$$.

6.P: Determination of Kc for a Complex Ion Formation (Pre-Lab) is shared under a not declared license and was authored, remixed, and/or curated by Santa Monica College.