[stephenball70]

Here I am, calculating these pH, pKa, and Ka values, right. I've gotten the hang of all the math involved, but I have serious issues with the theory behind how it's supposed to work.

We're given this equation, that pOH = 14 - pH, which implies that something that is very non-acidic should automatically be highly basic. Giving a solution a ridiculously high pH value of, say, 15 implies that it has a ridiculously low amount of hydronium in it. That's fine with me; the problem comes with its implications about the amount of hydroxide.

My Organic Chemistry for Dummies book says that CH4, methane, has a pKa value of 50. Makes sense, considering it's stable as a motherf*#$&@ when it comes to not randomly losing its protons. But in my inorganic chemistry book, we're told that Kw(E-14) = Ka X Kb. This implies that the Kb value of methane is absurdly low. This doesn't make sense, does it? Methane is neither very acidic nor basic as far as I know.

For methane, you could also convert that pKa into Ka, then convert that into pH and get a value of like 25 (I think), and a corresponding pOH of -9, which is completely impossible.

So what am I doing wrong here? Is it a problem with the theory, or a problem with how I'm interpreting it?

[Borek]

For methane, you could also convert that pKa into Ka, then convert that into pH and get a value of like 25 (I think), and a corresponding pOH of -9, which is completely impossible.

Show how you do the "conversion". You are omitting something obvious.

[Dan]

We're given this equation, that pOH = 14 - pH, which implies that something that is very non-acidic should automatically be highly basic.

No. This is a common misconception I have had to drive out of several of my student in the past.

If we consider the equilibrium:

AH A^- + H⁺

Let's say the pKa of AH is X, this tells us that:

The pKb of A^- is 14-X, not the pKb of AH is 14-X

Information about the acidity of AH can be used to calculate the basicity of its conjugate base, A^- (and vice versa). However, the acidity of AH and the basicity of AH are not directly related. Methane is an extremely weak acid and an extremely weak base. On the other hand, amphoteric compounds are reasonable acids and reasonable bases.

Example: H₂O has a pKa of ~16 (weak acid). This indicates that HO^- (not H₂O) is a strong base (pKb -2). The pKb of H₂O is also ~16 (a weak base) and the hydronium ion H₃O⁺ is a strong acid with a pKa of -2.

[stephenball70]

The pKb of A^- is 14-X, not the pKb of AH is 14-X

Ah, of course! I'm not sure what I was thinking when I said that. CH₃^- would be the one with the ridiculously high pKb.

It makes a bit more sense now. Thanks!