Continuing the discussion from The Trump Administration: The Clusterfuck Continues:
I thought this was too much of a hijack to continue in the linked thread.
This is a common misconception but is nevertheless incorrect.
pH = -log10[H+(aq)]
Therefore a 10 M solution of HCl has a pH of -1 and a 10 M solution of NaOH has a pH of 15.
Cite (pdf)
While I don’t disagree that pH can and does go negative, I’m not convinced that a 10M solution of HCl actually does disassociate into 10M hydronium ions.
It might…but it might not. I don’t recall enough from my education to remember if concentrated HCl actually does fully disassociate
HCl is a strong acid, so it does in fact completely dissociate in water. Also, per my cite above, commercially available concentrated HCl solution (37% by mass, which corresponds to a 12.2 M solution) has a pH ≈ -1.1.
Yes, I understand its a strong acid, and in ‘dilute’ solutions it will fully disassociate. But beyond General Chemistry, and into the realm of Physical Chemistry, concentrated solutions don’t fully disassociate. The ‘activity’ of concentrated HCl, may effectively keep the hydronium ion formation significantly lower than 12.2 or 10 M.
I just don’t remember enough of it to know to what extent HCl at 37% disassociates into the -Cl and H3O+
My understanding is that even concentrated solutions of HCl will fully dissociate (or ionize, to be more precise). This is supported by experimental evidence. The predicted pH of commercially available concentrated HCl solution (37% by mass, which corresponds to a 12.2 M solution) is a pH of -1.1, and the actual measured pH ≈ -1.1. (The approximation is because it is difficult to calibrate a pH probe at the extreme ends of the pH scale.)
If the HCl did not fully dissociate, the measured pH would be higher. We’d also have to come up with a new explanation for the behavior of strong acids in water.
As an aside, it appears that something in the range of 38-40% HCl by mass is the maximum amount of HCl you can dissolve in water. (Note that pure HCl is a gas at room temperature and pressure with a boiling point of -85 deg C. And like any other gas, there is a maximum amount that you can dissolve in water.)
In any event, being a strong acid, any HCl that dissolves in water will fully dissociate in water. Even highly concentrated HCl. Which again is supported by experimental evidence.
It might help to compare the situation to a salt like NaCl. Of course there is a maximum amount of salt that you can dissolve in water. However, whatever NaCl dissolves in water will fully dissociate into sodium ions and chloride ions. So even a highly concentrated solution of NaCl fully dissociates, even if it does not all dissolve.
In the case of a concentrated solution of HCl, any HCl you try to add beyond the maximum solubility of HCl bubbles out of solution, because pure HCl is a gas. Nevertheless, whatever amount that dissolves also fully dissociates.
You can contrast this with a weak acid like HF, in which the hydrofluoric acid only partially ionizes in water. In this case, you have aqueous HF molecules as well as hydronium ions and fluoride ions in solution.
My understanding is that aqueous HCl molecules simply do not exist—even in a concentrated solution, any more than aqueous NaCl molecules. Instead, the HCl molecules either fully dissociate/ionize, or remain a gas that will bubble out of solution if it is beyond the solubility limit.
While we’re at it, the core of the Sun has a pH of around -5.
Man, this litmus paper is fantastic!
I thought you were just being funny, but apparently not.
(Well, it is kind of silly, because pH is only defined for aqueous solutions, but it’s a fun exercise regardless, so kudos.)
I love the Dope. Up until now the idea the Sun had a pH had never occurred to me. Bravo @Chronos.
Which of course raises questions about the pH of the lower layers of gas giant planets. It’s liquid down in there somewhere. Or can pH only really be defined in the context of “dissolved / dissociated in liquid water?”
TIL today: for some highly concentrated acids, where the active molecule is something other than H3O+, pH is not an appropriate measure of the acid strength. Other measures have been devised for such acids, like the Hammett acidity function, called H0. At low concentrations, H0 is approximately equal to pH. The lowest measured H0 value is for fluoroantimonic acid, at H0 = -28.