# Keq puzzle

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1.Consider a reaction mixture that has initial concentrations of Fe3+ = 0.0050 M and SCN– = 0.0050 M. Without doing any calculations, which of the following values do you know?

a) The equilibrium concentrations of Fe3+, SCN– , and FeSCN2+

b) The sum of the equilibrium concentrations of Fe3+ , SCN– , and FeSCN2+

c) The product of the equilibrium concentrations of Fe3+ , SCN– , and FeSCN2+

d) The ratio of equilibrium concentrations of products to reactants: [FeSCN2+]/[Fe3+][SCN– ]

e) The ratio of equilibrium concentrations of reactants to products: [Fe3+][SCN– ]/[FeSCN2+]

Earlier in the experiment, a calibration curve measuring absorbance on the y-axis and concentration on the x-axis was generated from a set of 3 standard solutions. So, I am thinking perhaps the d and e are known due to a concentration/absorbance ratio that graph?

Next is this question,

Consider a reaction mixture that has an initial concentration of FeSCN2+ = 0.0050 M, no Fe3+ or SCN. Without doing any measurements or calculations, which of the following two values do you know?

a) The equilibrium concentrations of Fe3+, SCN– , and FeSCN2+

b) The sum of the equilibrium concentrations of Fe3+, SCN– , and FeSCN2+

c) The product of the equilibrium concentrations of Fe3+, SCN– , and FeSCN2+

d) The ratio of equilibrium concentrations of products to reactants: [FeSCN2+]/[Fe3+][SCN– ]

e) The ratio of equilibrium concentrations of reactants to products: [Fe3+][SCN– ]/[FeSCN2+]

By the way the equation for both is , the equation is Fe3+ (aq) + HSCN (aq) <-----> FeSCN2+ (aq) and everything is 1:1. Using the ice table, Fe3+ is 0.0050M -x , SCN-, is 0.0050M- x. So, we know a and b?

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1 hour ago, gammagirl said:

Earlier in the experiment, a calibration curve measuring absorbance on the y-axis and concentration on the x-axis was generated from a set of 3 standard solutions. So, I am thinking perhaps the d and e are known due to a concentration/absorbance ratio that graph?

Absorbance of what, exactly?

1 hour ago, gammagirl said:

Using the ice table, Fe3+ is 0.0050M -x , SCN-, is 0.0050M- x. So, we know a and b?

How do you figure that? They haven't said that the FeSCN2+ was made from those two ions, just that there was a flask that contained FeSCN2+. Besides, this is a reaction that exists in equilibrium and does not go to completion (in addition to the fact that they said that it was an initial concentration of FeSCN2+). Perhaps try writing the reaction in the other direction.

You may find this useful: http://mctcteach.org/chemistry/C1152/Laboratory/Lab_Protocals/Det_of_Equil_Const_v.1.16.pdf

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Posted (edited)

Absorbance of FeScn2+

And the link IS the experiment

And that is the point the reaction is in equilibrium but the questions are confusing. If The Fe3+ was in excess, then Scn-=FeScn2+. But for the first question question, Fe 3+=Scn - is the same amount (as in the lab).

It is something intuitive and easy.

Edited by gammagirl

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I definitely agree that the questions are confusingly written. I will think on it more and get back to you tomorrow, though you may need to contact your professor or TA.