[armless]

Which ionic compound forms a pH-neutral aqueous solution at 25C?
a. Na2Co3
b. KNO3
c. LiF
d. NH4Cl
e. K2S
I know the answer is B, but don't understand why. Thanks!

[Fluorine]

KNO₃ dissociates into K⁺ and NO₃^-. NO₃^- is a conjugate base of a strong acid. Strong acids have very weak conjugate bases; this is why HF is a weak acid. So it's as if putting a very weak base into solution.

The same couldn't be said about F^-, NH₄⁺, S^2-, CO₃^-2...assuming "Na2Co3" is not a cobalt compound. Carbonate formula is CO₃ not Co3, as Co is cobalt. If you wrote this on a test you'd lose points.

[Borek]

KNO₃ dissociates into K⁺ and NO₃^-. NO₃^- is a conjugate base of a strong acid. Strong acids have very weak conjugate bases; this is why HF is a weak acid. So it's as if putting a very weak base into solution.

I guess you may want to reword this part of the answer. There is something wrong.

[Fluorine]

I can't find what's wrong - is it the last line?

[Borek]

Strong acids have very weak conjugate bases; this is why HF is a weak acid.

I can't see the logic behind this phrase. I guess you meant F^- is a relatively strong base, so HF is a weak acid, but it is not clear, besides, you have not stated that it works both sides, so the conclusion doesn't follow information given.

[Fluorine]

I see, English isn't my first language so it's not always easy to articulate thoughts clearly. That is what I meant, HF has a strong conjugate base, F^-, so it is a weak acid as result.