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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Araconan on July 17, 2012, 11:17:32 AM

Title: Question Regarding Equilibrium Problem
Post by: Araconan on July 17, 2012, 11:17:32 AM
I am currently attempting the following problem:

At a certain temperature K = 1.1 x 103 L/mol for the following reaction:

Fe3+(aq) + SCN-(aq) <-------> FeSCN2+(aq)

Calculate the concentrations of Fe3+, SCN-, and FeSCN2+ at equilibrium if 0.02 moles of Fe(NO3)3 is added to 1L of 0.01M KSCN. (Neglect any volume change)

My attempt:

With the provided amounts of compounds, once placed in an aqueous solution, there will be a concentration of 0.02M Fe3+, and 0.1M SCN-.

Constructing an ICe table:
                           Fe3+(aq) + SCN-(aq) <-------> FeSCN2+(aq)
Initial:                 0.02M         0.1M                      0M

Change:              -x               -x                           +x

Equilibrium:        0.02 - x       0.1 - x                      x

The quadratic equation would then be:

x/(0.02-x)(0.1-x) = 1.1*103

Simplifying to:

1100x2 - 113.2x + 2.2 = 0

Solving for x, the two values would be

x1 = 0.0769
x2 = 0.026

However, both values can't be correct since I only have 0.02M of Fe3+. Even if I round 0.026 to 0.02, I would get 0M, which isn't what the answer states. (Answer being 2*10-4)

So where did I go wrong?

Thank you in advance!
Title: Re: Question Regarding Equilibrium Problem
Post by: Borek on July 17, 2012, 11:38:37 AM
Quote from: Araconan on July 17, 2012, 11:17:32 AM
Equilibrium:        0.02 - x       0.1 - x                      x

Check your math.
Title: Re: Question Regarding Equilibrium Problem
Post by: Araconan on July 17, 2012, 10:38:52 PM
Ahh, that awkward moment when after double checking my work, I still make an error  :D
I see where I went wrong now, equation should've been 1100x2 - 133x + 2.2 = 0

Thank you!