[Darwin]

Hi

I have a question that has bothered me for long now, and I can't really find an answer. For an e.g. HCl(aq) solution, is the following relation true?

a_HCl² = a_H⁺*a_Cl^-

or should it rather be a_HCl = a_H⁺*a_Cl^-?

where a is the activity.

[Schrödinger]

Hi

I have a question that has bothered me for long now, and I can't really find an answer. For an e.g. HCl(aq) solution, is the following relation true?

a_HCl² = a_H⁺*a_Cl^-

or should it rather be a_HCl = a_H⁺*a_Cl^-?

where a is the activity.

What seems logical to you and why?

[Darwin]

What seems logical to you and why?

Well, normally for a reaction like HCl   H⁺ + Cl^-, we write [HCl(aq)] = [H+] = [Cl-], so [HCl]² = [H+][Cl-], and the exact same thing with molalities. So my logical answer is that a_HCl² = a_H+a_Cl-. However, in a standard solution book that I have, it is indicated that a_HCl = a_H+a_Cl-, though without any concomitant explanation. I'm wondering if that's wrong..

[Borek]

I am not aware what a_HCl is, unless it is activity of dissolved but undissociated HCl, similarly to - say - a_CH3COOH - activity of undissociated acetic acid. Usually we assume activity of uncharged species is just their concentration (activity coefficient is 1).

[Schrödinger]

Well, normally for a reaction like HCl  H⁺ + Cl^-, we write [HCl(aq)] = [H+] = [Cl-], so [HCl]² = [H+][Cl-], and the exact same thing with molalities. So my logical answer is that a_HCl² = a_H+a_Cl-.

Why do think it is [HCl]² ?