[zmasterflex]

Greetings all. The question asks "The Ksp for Al(OH)3 is 2.0e-31. What is the value of delta G for the precipitation of AL(OH)3 at 25 C?

This is what I came up with; delta G = -RTlnKeq

To calculate Keq, I got the "constant" 27x^4. I then solved for x^2 and plugged it into the equation. The answer I got doesn't come close to any of the given answers. Am I even using the correct equation? This is an MCAT review question and there are no calculators on the MCAT. Thanks

[Corribus]

Where are you getting your value of Keq from?

[zmasterflex]

I really don't know. I thought that the constant for Al(OH)3 is 27x^4. Therefore if I solve for "x" then I have the amount of each ion in solution. Keq will be x^2. Or is it something else? (I'm completely lost)

[Corribus]

Ok. Keq is just a generic equilibrium constant. Ksp is an equilibrium constant for a particular type of equilibrium. You don't really have to solve for this since you're given the equilibrium constant up front. Do bear in mind that they seem to want the ΔG for precipitation reaction, not the dissociation reaction, so you'll have to be careful with your signs. It will be helpful for you if you write out what the actual reaction is, and what the reaction is for the equilibrium constant that they give you.

[zmasterflex]

(Thanks for helping me)
The reaction is; Al3+(aq) + 3OH-(aq) ---> Al(OH)3(s)

The Keq for this reaction is equal to the Ksp. expression = [Al3+][OH-]^3
How do I continue from here?

[Corribus]

You are given Ksp so you don't have to calculate it. What is Ksp defined as? (i.e., what reaction/process)

Btw, what are the choices given for answers?

[zmasterflex]

So what you are saying is that the Keq here is really the same as the Ksp, therefore all I need to do is plug in the given value into the formula. Because the value is a dissociation constant and the question wants precipitation I need to switch the sign on the delta G. (The sign on the Ksp remains the same either way)

This is what I'm taking out of this back and forth between us. I'm I getting it right?
The answer given is -175 KJ/mol
My calculations get something else

[Corribus]

Show me how you are doing your calculation.

[zmasterflex]

Ahh. I got it. I was using the wrong R value. The review course gives a quick memory technique --> RT is approx 24. The R here is a different R.
Delta G = -RTlnKsp = -(8.314)(298)ln(2e-31) = -(-175,132.1089) = positive 175KJ.
However the question wanted the Delta G for precipitation so we switch the sign to negative. Thanks for the help

[Corribus]

No problem.