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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: Mager on September 17, 2017, 01:33:34 PM

Title: Solving Equation for Final Step of Systematic Treatment of Equilibria
Post by: Mager on September 17, 2017, 01:33:34 PM

I am having quite a bit of trouble in solving for the concentrations of [Zn2+], [CN-], and [HCN].  I have 3 equations and 3 unknowns yet I can't seem to work the algebra out correctly.  It's also possible I have done it correctly and don't realize it.  I am hoping someone can please verify if I have performed the algebra effectively.  The reason the charge balance is not being used is because it is invalidated by a fixed pH. I am using the Ksp, Kb, and Mass balance.  Any help would be greatly appreciated as I've spent a significant amount of time (much more than I should have) in attempting this.  It's a matter of pure algebra and solving for the unknowns.

My attempt (at least 1 one of many) is attached.

Title: Re: Solving Equation for Final Step of Systematic Treatment of Equilibria
Post by: chlorinecrown on October 03, 2017, 11:10:17 AM

Here's what I do when things get this messy: start replacing all my constants with consolidated constants, like A=[OH-]/Kb, for instance.

I rearranged all the equations so that x=Ksp/y, y=Az, z=B/z²-Az, by using the first two substitutions in my equation for z. (B=2(KspKb²)/[OH]-).