4
$\begingroup$

I'm quite a newbie in chemistry and I'm trying to learn how logarithmic diagrams work. I have been told that you can calculate pKa and pH graphically:

  • pKa, when [HA] = [A-]
  • pH, when [H+] = [A-]

Why does this happen?

enter image description here

$\endgroup$
  • 3
    $\begingroup$ $\mathrm{p}K_{\mathrm{a}} = \mathrm{pH}$ when $\ce{[HA]} = \ce{[A-]}$. This is easy to see from the form of the Henderson-Hasselbalch equation. So, we the concentration of hydronium is equal to concentration of conjugate base, you can read off the $\mathrm{p}K_{\mathrm{a}}$ from whatever the $\mathrm{pH}$ reads. It's not clear what you're second statement means... $\endgroup$ – Zhe May 16 '17 at 21:24
1
$\begingroup$

You should state the charge balance or the proton balance.

[H3O+] = [OH-] + [CH3COO-]

We take the logarithm of this and get: log [H3O+] = log([OH-] + [CH3COO-]).

Now, you can plot this in your nice diagram.

log [H3O+] will follow the straight line in your diagram, i.e. log [H+].

The line for log([OH-] + [CH3COO-]) will at very high pH follow the line for log([OH-], but quite soon it will bend off and only follow the line for log [CH3COO-]. This is because [OH+] << [CH3COO-] at lower pH.

When [OH+] << [CH3COO-], you get log [H3O+] = log [CH3COO-]). When the line for log [H3O+] crosses the line for log [CH3COO-], you can calculate the pH of the solution from the diagram. The lines are crossing each other at right angel: pH = (1.00 + 4.65)/2 = 2.82(5). Very good, I am encouraging you to use logarithmic diagrams for solving acid-base equilibria.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.