Problem: 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.

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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:


Kf = [products][reactants]=[FeSCN2+][Fe3+][SCN-]


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+.


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Problem Details

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.

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