Hi Moiz!



Q139: I'm curious to see how you calculated your answer and get B. :)



Recall that Kw = [H30+][OH-], and that we have 1 M HCl. When we dissolve HCl in water, the following reaction occurs:



HCl + H2O --> H3O+ + Cl-



Since HCl is a strong acid, we can assume that it totally dissociates, so 1 M HCl will give 1 M H3O+.



At 10°C, Kw = 0.292 x 10^-14. Rearranging and solving for [OH-], we have



[OH-] = Kw / [H3O+] = (0.292 x 10^-14) / 1 = 0.292 x 10^-14 = 2.92 x 10^-15



This is closest to answer B.



Q142: This one is tricky. Both acetic acid and water act as BASES when compared to HCl and HClO4 because acetic acid and water are both weak acids in comparison to those two strong acids. When we want to make a distinction between HCl and HClO4, we need to add these two strong acids to a very weak competitor.

I imagine it like a boxing fight. I want to have HCl "fight" with someone and have HClO4 also "fight" with that same opponent. I DO NOT WANT the fights to be close. If the fights are close, then I won't be able to tell who is stronger between HCl and HClO4. But, if I choose a super weak base, then I'll be able to tell who is the best acid based on the "post-mortem". (This is getting dark).

So, we choose a really bad base (which would be a decent acid).

I hope this explanation helps!



Q151: Weirdly, no! The first neutralization is happening on the first "vertical" section, and the second neutralization is happening on the second "vertical" section, so this represents the titration of a DIPROTIC acid.



Q156: Here, we're not strictly calling them acids (because, as you said, they're not "acids"), but comparing them on a "relative" basis. Which more is MORE acidic than the other. Since the lysine side chain has a lower pKa (10.5) than the arginine side chain (12.5) (which you are just supposed "to know"), then the lysine side chain is RELATIVELY more acidic than arginine's.



It's kind of like comparing McDonald's to Arby's. Both AMAZING choices for your health. 🤨 Which one is healthier? Hmm...



Q161: This one's sneaky. In aqueous solution, NaH becomes Na+ + H-. Notice that the H- has an extra set of electrons to give away. OOOOH LEWIS BASE. 😮

HCl is a strong acid (one of the ones that we need to memorize!), so it MUST dissolve in water.

When you get to work with HCl in the lab, you will get to work with strong acids and bases quite a bit more. Until it is mixed with water, HCl (hydrogen chloride, which is actually a gas), doesn't begin to behave as an acid! We need to mix it with water before it actually "works".

So, I'm introducing this idea to you (mixing it with water), because you physically need to do that the product in real life.

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Water is a good solvent for many different molecules because it has the ability to hydrogen bond, it is polar molecule, and it has polar bonds. For molecules that are bonded ionically (ionicly?), it will help them dissociate readily! It can also "shield" the newly-formed ions in a process is called "solvation".

Here an example of water shielding a Na+ ion: https://upload.wikimedia...1200px-Na%2BH2O.svg.png

These substances will all readily dissolve in water due to water's ability to "solvate" the ions.

The ion pairs are as follows:

Na+, H-

NH4+, Cl-

Fe3+, Cl-

H+, ClO3-

You're right, they all do have negative charges, but it's pretty WEEIIIIRRRRRDDD to see hydrogen with a negative charge. So it REALLY wants to get rid of that negative charge.
And by definition, a Lewis base is an "electron donor". So, who is the most likely to donate their electron? The H- ion, because it is VERY UNSTABLE in this state.
The other ions (Cl- and ClO3-) are decently stable like this, and so are less likely to donate their electrons when compared to H-.
Out of all of the answers, H- is the best Lewis base.