Molarity and Strong Acid Ionization

[Sofia Lucido 3L]

In today's lecture Dr. Lavelle talked about how because we can approximate that strong acids are 100 percent ionized, if we are given a 0.1 M HCl aqueous solution, then we can say that we will have 0.1 M of H3O+ and 0.1 M of Cl-. I am really confused on this. Can someone explain how this works out?

Thank you!

[Arielle Sass 2A]

Hi!
In order to determine pH, we have to determine the concentration of H+ ions, which acids dissociate into (as well as some other anion). Because strong acids are characterized by releasing a lot of H+ ions, strong acids break apart very easily in water (since the pull of the dipole of the water molecule is strong enough to pull the weakly bonded acid apart and take the H+). So, when we're doing calculations with strong acids, we can assume that ALL of the acid dissociates into its component ions when it is in water. For example, HCl is a strong acid so when it's put in water, we assume that ALL the HCl molecules break into H+ (which bonds to H2O) and Cl- ions. On the other hand, HF is a weak acid so only some of the molecules would break into H+ and F- ions and the rest would stay as HF molecules.
If there's a 0.1 M solution of HCl, and each HCl molecule breaks into its component ions, then we will have one H+ ion and one Cl- ion for every HCl. The H+ ions attach to H2O because of the dipole on it, so H2O becomes H3O+.

I wasn't exactly sure which part of it you were confused about so hopefully this description explains what you were missing!

[Jenny Lee 2L]

Hi Sofia! In that example, the strong acid, HCl, was 100% ionized, which means that all HCl (aq) molecules dissociated into H+ and Cl-. However, H+ is just a shorthand for H3O+, because the H+ bonded with the water (hence why HCl is denoted aqueous). So, you can just see HCl (aq) --> H3O+ + Cl-.

You can try thinking of the next part like this:
1. Remember that for example, if we have 3 H+ atoms and 3 Cl- atoms, we can make 3HCl molecules.
2. Similarly, if we imagine that we have 0.1 moles of H3O+ (same thing as 0.1 moles of H+) and 0.1 moles of Cl-, we can make 0.1 moles of HCl molecules.
3. Remember, Molarity = # moles of solute (HCl) / total liters of solution.
4. So if we have 0.1 M (molarity) HCl, we will have 0.1 M H3O+ and 0.1M Cl-. Remember that we didn't change the liters of solution (the denominator).
5. You can just visualize that HCl split into H+ (to create H3O+, so same thing) and Cl- in a bottle of water.

Hope this helps!

[Joseph_Armani_3K]

HCl, as a strong acid, will dissociate in water, and the H+ ion will be accepted by H2O to create H3O+. The acid-base interaction is as follows:

HCl + H2O <--> H3O+ + Cl-

All it takes from here is stoichiometry. The ratio of HCl to H3O+ is 1:1, same with HCl to Cl-. That's why there is 0.1 M H3O+ and 0.1 M Cl- for 0.1M HCl. Hope this helps!