Potassium thiocyanate, KSCN, is often used to detect the presence of Fe3+ ions in solution by the formation of the red Fe(H2O)5SCN2+ (or, more simply, FeSCN2+). What is the concentration of Fe3+when 0.52 L each of 0.0012 M Fe(NO3)3and 0.21 M KSCN are mixed?Kf of FeSCN2+ = 8.9 x 102.
We’re being asked to determine the concentration of Fe3+ when Fe(NO3)3 and KSCN are mixed.
Since the compounds are ionic compounds, they form ions when dissociating in water.
The dissociation of Fe(NO3)3 and KSCN in water are as follows:
The nitrate ion, NO3–, has a charge of –1. Iron then has a charge of +3:
Fe(NO3)3 → Fe3+(aq) + 3 NO3–(aq)
The thiocyanate ion, SCN–, has a charge of –1. Potassium is in Group 1A so it’s charge is +1:
KSCN → K+(aq) + SCN–(aq)
When Fe3+ and SCN– are mixed, the red complex FeSCN2+ is formed:
Fe3+(aq) + SCN–(aq) ⇌ FeSCN2+
The equilibrium constant for this is called the formation constant (Kf) and is associated with the formation of a compound from two ions. Kf is also the opposite of Ksp.
The Kf expression for this is:
Note that each concentration is raised by the stoichiometric coefficient:
[Fe3+], [SCN–], and [FeSCN2+] are raised to 1.
We need to do the following for this problem:
Step 1: Calculate the moles of Fe3+ and SCN–.
Step 2: Construct an ICE table for the reaction.
Step 3: Calculate the equilibrium amount of Fe3+.
Step 4: Calculate the equilibrium concentration of Fe3+.