Ignoring change in initial weak acid concentration

[kcola]

At what Ka/pKa values can you rightfully ignore change in concentration of weak acid in the the dissociation/equilibrium equation?

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[kcola]

Oh, i guess the answer is when the Ka is closer to 1.

Also, would there be any great chance an MCAT question would require us to find concentrations of H/OH when we need to take into account H+ from dissociation of H2o, using the quadratic formula?

 

[Rabolisk]

Don't you mean when Ka is far from 1?

The real answer is that you can ignore it if it doesn't change any significant figure in the concentration of the weak acid. If you have Ka = x^2/[HA]-x, then you can ignore the -x if it doesn't affect [HA]. E.g. if your initial concentration was 1.00, and x came out to be 0.00001, then 1.00 - 0.00001 = 1.00, so you can ignore it. My recommendation is to ignore it and then back-substitute it to see if it produces any deviation.

I don't quite know what you mean by using the quadratic formula, but let me give you one tricky question. What is the pH of 10^-8 M solution of HCl?